add IntelliJ-IDE-generated overlay directories to .gitignore

IntelliJ IDE can helpfully generate directories reflecting the overlaid-upon `.war` files.
These directories should not be committed to source control (being generated from source) and this commit enhances `.gitignore` to help developers to properly ignore these directories in interacting with source control.
Merge pull request #199 from Jasig/gitignore_intellij_overlay_directories

add IntelliJ-IDE-generated overlay directories to .gitignore