Add .idea to .gitignore file to work with PyCharm IDE

The company JetBrains got really nice IDEs for various programming languages.
One IDE is PyCharm, the Python IDE.

I use the community version of PyCharm for Python development: http://www.jetbrains.com/pycharm/
If you open a folder / project with this IDE, the program will create a .idea folder to store meta information.

This PR just add this .idea folder to .gitignore file