update gitignore to better support AndroidStudio

cleanup all gitignore files
remove redundant iml file
I did't come up with this config. This one is used by very skilled Android devs, I think @JakeWharton initially came up with it.
I've got a theory why this two rules are declared separately. 
It's considered fine to add .idea/ to repository, it can simplify development if whole team uses IntelliJ Idea. However event in that case workspace.xml should be ignored since it's different for every environment.