Change .gitignore to not ignore several Intellij project files.

References
http://stackoverflow.com/questions/11968531/what-to-gitignore-from-the-idea-folder
see also Intellij recommendations
http://web.archive.org/web/20120629000805/http://devnet.jetbrains.net/docs/DOC-1186
Fix .gitignore syntax
Commit newly un-ignored Intellij project files.