Add logging using NLog 2.0 & upgrade to .net 4.0

Implemented DEBUG and TRACE levels in NLog.
commiting .gitignore. Forgot to stage it.
Remove unnecessary check for no replacement file & bump to version 0.6.4 which includes tagging
Add NLog.dll to the depenencies folder. NLog.dll must be in the same directory as kgrep or in the path.