msgstr and msgid nullpointer

Unfortunately I added nullpointer errors for msgid and msgstr on obsolete msgs with my last pull request. These could break some existing code. I fixed it with this commit.

Anyway I would highly recommend removing the 0.6.4 version tag, and going to 0.7.0 as this updated functionality could still lead to misbehaviour on existing code. e.g. an empty msgstr doesn't automatically mean a missing translation anymore, but could also be an obsolete translation. It could be necessary to .reject obsolete translations like plural/fuzzy in some cases. Adding an "obsolete" option on parsing could be a good idea, but I'm not sure on the best way to add this as it would mess with the current static interface.

I added IDE and rvm files to the gitignore. Don't merge it with it bothers you :).