support 'go get', 'go install'

[jpillora]

First off, awesome project 👍

This PR doesn't change any files (except for package import paths), it just restructures the directories to allow for use in a standard Go environment, that is, a global GOPATH. It also makes use of the vendor/ directory to support the included dependencies. Most gophers set their GOPATH in their shell profile, and optionally add $GOPATH/bin to their PATH. This results in:

First time use:

# fetch github.com/twitchyliquid64/subnet into GOPATH/src/github.com/twitchyliquid64/subnet
# and compile the target main package into GOPATH/bin/subnet
$ go get github.com/twitchyliquid64/subnet
# with $GOPATH/bin in our PATH, we can then
$ subnet --help

Development:

$ cd $GOPATH/src/github.com/twitchyliquid64/subnet
# hack away....
$ go install
# compiled to GOPATH/bin/subnet
$ subnet --help

[twitchyliquid64]

This is awesome, thanks!

I'm not sure how to use this PR interface or exactly what the etiquette is, but I noticed the commit you have so far deletes a bunch of code (presumably to move it to the correct path) but doesn't add it back anywhere (4 lines added, ~2k lines deleted)? I guess theres another commit coming and I should sit-tight?

[jpillora]

Ah yes, my mistake. The .gitignore had /subnet in there. I should have caught that! Now "Files Changed" tab is mostly: File renamed without changes.. A few notes:

• github.com/twitchyliquid64/subnet is the main package (when a repo represents a cli program, I make the root dir the main package and I put the library code in a subdir).
• github.com/twitchyliquid64/subnet/subnet is the library package for programmatic usage.
• github.com/twitchyliquid64/subnet/vendor is the dependency dir. it is special since Go resolves pkg by first checking ./vendor/<pkg> then checking $GOPATH/src/<pkg>.

(I did a git push --force to keep the history a bit cleaner)