v1.2 (Rails 4 compatible)

Hi,

It's a collective PR bringing lot of fixes and tweaks added throughout the time, including, but not limited to:
- Rails 4 support #34
- Support strings or symbols in `distance_of_time_in_words` helper
- Support for `highest_measuress` in above mentioned helper
- Better I18n support #17
- Returning "Just now" (configurable) if two times are the same #28 
- Code modularization
- Misc. fixes and stuff

Tests are failing due to some bug in calculations which I admit, haven't addressed. it does seem to come from upstream though (see comments in #34), so one might say it's heritage, family thing, we all know that ;)

I hope you'll find it helpful!
Let's change this key to be what `rails-i18n` has: https://github.com/svenfuchs/rails-i18n/blob/master/rails/locale/en.yml#L55-L57