Add conversion method, TimeOfDay().

Behaves similarly to ruby's Array() method - aims to do the sensible thing
with a variety of inputs.
Make time of day compatible with MongoDB

This should probably actually be in a rails-tod gem (cf. rails-money).
As it is, in order to test I would have to have a dummy rails app in the,
library, which doesn't seem right ...
Fix error on mongoid load when object is nil
Merge pull request #12 from tomclose/master

Add conversion method, TimeOfDay().