Adding Apache license to haxelib tool.

Reasoning:
 * The Apache 2.0 license is quite often used, according to Ohloh/Blackduck it is the #3 in 2014 at 16% overall usage. This is more than GPL 3, BSD 2 or any of the LGPL. Only MIT and GPL 2 do have a higher usage.
 * There were no significant arguments against it in the thread [2] from 2010 other than the intention to avoid a "license hell", which is of course understandable, but does obviously no longer apply to the Apache 2.0 license.

[1] https://www.blackducksoftware.com/resources/data/top-20-open-source-licenses
[2] http://old.haxe.org/forum/thread/3395#nabble-td5546987