NSIS 2.21

Major Changes
- Added WinVer.nsh for easy Windows version comparisons
- Upgraded to lzma sdk 4.43 for faster compression

Minor Changes
- Added Vista support to GetWindowsVersion
- Added x64.nsh including a few simple macros for handling x64
installations
- Fixed a handle leak in Locate
- Minor documentation updates and fixes
- Modern UI 1.76: Added MUI_ABORTWARNING_CANCEL_DEFAULT (RFE #1547844)

New/Changed Commands
- Added RequestExecutionLevel (RFE #1524709)

Translations
- Added Valencian translation (patch #1558822)
- Bulgarian corrections
- Slovenian corrections

Build System
- SCons requirement upgraded to 0.96.92