Add Income categories

[PawanHegde]

Fixes https://github.com/sameersyd/Expenso-iOS/issues/31 by adding separate categories for incomes.

IMO users should be able to add/customise their own categories without needing to update code, but we can do it later if the maintainers of this project would be interested in it. :)

I've not added any new icons yet. I can add them if the code looks good. Also, because I'm not sure what would be good icons for these new tags.