Download dependencies from GIT (and haxelib) with a new entry : "requirements"

Hello,

This PR introduce a new way (more flexible) to declare dependencies in haxelib.json file
To not break current libraries, I added a new key "requirements", which is an array that contains informations about these dependencies.

You can have a look to a test JSON file here : https://github.com/Peekmo/Peekmo-haxe-tools/blob/master/haxelib.json

Why ? Because in a company, you don't always want to share your libraries on haxelib, and for moment, that's impossible to download dependencies from a GIT project, and more, that's impossible to declare dependencies to git project inside a haxelib project. And that's my need, to be able to share my tools without adding them to haxelib (because they are not really interesting outside my projects).

My idea is (perhaps) not the best, but with this kind of declaration, we could support subversion for example (just for example XD), or something else, who know the future ? 

The "version" for a git project => git tag

Thank you for your reading.