Fully qualified path to powershell.exe

Having just been installed, powershell is not in the default path during a 2008 R2 Core install. This fixes issue #50.
compact.bat optimises files and zeroes free space

compact.bat uses the ultradefrag and sdelete tools to optimise files on disk and zero the free space afterwards, which results in smaller boxes.
Use compact.bat for 2008 R2 Core images
Use compact.bat for 2008 R2 images
Use compact.bat for 2008 R2 images
Use compact.bat for 2012 R2 Core images
make sure we have 7zip in compact.bat
Merge pull request #53 from jdub/compact

Use ultradefrag and sdelete to reduce the size of generated boxes
Merge pull request #52 from jdub/2008_r2_core_fixes

Fully qualified path to powershell.exe
Update CHANGELOG
Merge branch 'release-1.14'