Add support for php7.1 and up.

This is to add return type and type hinting support.
Everything above is optional. This way we can make sure,
we only get the correct types without the need to check them.

Also drop symfony/console 2 support for dev requirements.

Updated beberlei/assert to version 3.x.x & PHP min version 7.0 (#32)

* Updated beberlei/assert to version 3.x.x
* Minimum PHP Support v7.0
* Moved beberlei/assert to require-dev and added additional directives in composer.json

Feature: inherit contracts (#31)

* Made AbstractFetcher accept more then one annotation type

* Add inherit annotation

This will serve as a contract inherit flag.
It will take all existing annotations (ensure, verify, invariant)
and check current as well as parent classes for contracts

* Update Inherit implementation

- Inherit will now always inherit contracts (despite missing @inheritdoc)
- provide Inherit tests
- update Demo implementation
- update Readme with Inherit documentation

Merge pull request #13 from lisachenko/simplify-code-after-changes

Simplify code after PR

Merge pull request #10 from pdaw/master

[Feature] Contract propagation for subclasses

Fixes after PR merge

Enable code-coverage, tests and badges

Use PSR-4 namespaces for better readability