Added PausableCrowdsale contract

[TalAter]

🚀 Description

Introduces a new contract, the PausableCrowdsale.

PausableCrowdsale extends the standard Crowdsale contract, making it pausable. This is a useful failsafe mechanism used by many crowdsales, but isn't yet offered by zeppelin-solidity.

• While paused, attempts to buy tokens using either buyTokens() or simple transactions will revert.
• The crowdsale can only be paused and unpaused by owner.

🦆🦆🦆🦆🦆🦆

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented.
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[come-maiz]

Thanks for the contribution @TalAter. This makes sense to me, because just pausing the token doesn't cover all the possible scenarios of crowdsales.
Let's wait for the second review by a fellow maintainer to confirm.

[TalAter]

Thank you @ElOpio

By the way, this code is currently used in an active Crowdsale that has already raised tens of thousands of Ethers. Plus the contract was audited by Matthew Di Ferrante. So that could count as a 3rd and 4th indicator once we have a second 😉

[TalAter]

Small update: our crowdsale which used this contract successfully closed last week raising over 40,000 Ether. No issues were found in this trial by fire.

[amateescu]

We should change the inheritance order to is Pausable, Crowdsale.

[TalAter]

Nice catch @amateescu
Updated the code. Thank you 👍

[TalAter]

@ElOpio could you re-approve this after the change done in 35e799c ?

Thank you.

[amateescu]

@TalAter looking at the Travis CI failures, I think the test coverage from this PR needs to be updated.

[TalAter]

Brought this branch up to speed with the latest changes done to HEAD.

• Merged all the latest changes from HEAD.
• Broke a line in class to comply with new solium linting max line length.
• Updated a Mock class I added to use the new constructor syntax.
• Updated tests to comply with new past-tense event names (introduced in Rename events to past-tense #1181)
• Updated tests to remove chai-as-promised and calls to should.be.fullfilled (changed in Remove chai-as-promised #1116 )
• Changed import to require in test (babel was removed in Remove Babel #1074)

[frangio]

Great work @TalAter! We're wrapping up the 2.0 release candidate this week so I'll get back to this in a few days.

[frangio]

Nice work @TalAter! I merged master into your branch in order to trigger an updated Travis run.

Left some comments. Can you also re-indent your files to 4 spaces? (We made that change recently in #1508.) Thanks!

[frangio]

Since we want to merge this for v2.1, I implemented the changes myself. (Sorry @TalAter!)

@nventuro can you review this now?

We need a third pair of eyes.