Ignore IntelliJ IDEA project files
Added rules for git to ignore IntelliJ Idea project files.
It ignores both project formats (Directory based -.idea, .iml- and File based -.ipr, .iws, .iml-)

Jira issue: WP-726
Merge pull request #449 from cbotsikas/WP-726

Ignore IntelliJ IDEA project files