Dropped changing CUrrentCulture to prevent "turkish i" problem (issue #45) as it is now correctly fixed in c35bb4c5f8cbafb8ee535dbf996dfa2c231bd5bb (issue #70)

Also psake.ps1 should not change it's environment in any way, it's rude:)
Also fix for crash introduced in previous commit (e49692ad3df1fd89411b8f6d3fcd705685efa4e5) when removing default value for $buildFile argument