Added "nme create extension <name>" and extended "nme rebuild" to support compiling user extensions.

If you want to create a new NME extension, open a commend-prompt and run "nme create extension MyCoolExtension". This creates a template with a simple sample method.

To compile, run "nme rebuild <target>" in the same directory, or "nme rebuild path/to/extension/directory <target>" or "nme rebuild <name of haxelib> <target>"

You can also use commas to denote multiple targets, like "nme rebuild my-extension windows,android,blackberry"