Install from hxml file

This is part 1 of the changes to "haxelib install".

The syntax as it stands is:

    haxelib install 		# asks for libname or hxml file
    haxelib install erazor	# install specific project
    haxelib install erazor 2	# install specific project/version
    haxelib install build.hxml  # install all libs and versions listed in build.hxml
    haxelib install all         # scan the current folder for hxml files, install all listed libs

I think as well as the changes I've already made, we should change the version syntax, and allow multiple installs:

    haxelib install mylib:2 otherlib:1.0.3 difflib:0.1

Thoughts?