Add PowerShell filetype

[Windows PowerShell](http://en.wikipedia.org/wiki/PowerShell) is a command-line shell and associated scripting language which comes out of the box with Windows 7/Server 2008/8/Server 2012.

Original lexer from Scintilla is used.

This feature was requested in [mailing list](http://lists.geany.org/pipermail/users/2009-October/005133.html)