Century and Decade precision

This is a first-pass on the feature that I'm trying to add.  I'm working on a project for the Carnegie Museum of Art, tracking the provenance (history of ownership) of art, and I'm using your library to handle storing the precision of the dates.

Unfortunately, museum data is woefully imprecise—often I only know what century something happened in, or the decade.  I've added some base methods to allow me to store that information using this gem—since there's no good way to write in computerese 'The fifteenth century", I'm manually setting the precision of those dates: `DateTimePrecision::DECADE` and `DateTimePrecision::CENTURY`.

Currently, the minimal changes I've made work for how I'm using it, but only done the bare minimum needed.  I don't know if this is something that you're interested in adding to your library—if not, I'm happy to keep a forked version for my own use.  If you are interested, I'd love a discussion of what's needed to bring these precisions up to the standard that you want for the library.

Initial thoughts on needed functionality include adding `to_century` and `to_decade` methods and string formatting for those precisions.