Support for scoped setups

[dpomier]

Allow using hxp from local scopes:

• This PR depends on Unify runProcess and runCommand #21, and changes Haxelib.runCommand and Haxelib.runProcess to always pre-concatenate the arguments. It allows haxelib to be run from a non-executable command (for instance with lix). The concatenation is pretty naive, so care needs to be taken when dealing with paths.
• HXML.buildFile is added and gives the possibility to build a hxml file from a given directory. To work this required to remove the comments from hxml files - see changes in fromString.
• A few calls have been updated to allow non-executables commands (namely neko, haxelib and haxe).

[EliteMasterEric]

Anyone aware of the status of this change?