Ignore node_modules, bower_components, dist, tmp, and sass-cache

sass-cache, dist, and tmp directories can be ignored since they will be changing when running grunt build tasks.

node_modules, bower_components, can be ignored because
1. We won't be changing any code there
2. There is no need to keep those files under git scm because they can be installed per user.
3. Some edge cases arise with things like bcrypt on osx vs linux, and letting contributors use their proper version will save us a lot of headaches.
4. The dependencies will be installed via build scripts on the server during deployment rather than scp'ing 24k files/folders over the wire.