Add 12/24 hour format with hours_fmt option and time_formatter

[hawkins]

Hey there,

I included a very simple fix for #37

Included hours_fmt option to opts and time_formatter method to handle formatting a date's time.

This change really only adds ' AM' or ' PM' to the end of the timestamp and adjusts the hours portion appropriately if the user set hours_fmt: 12 when passing in options.

I didn't touch the cases where Globalize exists. I assume that it would have its own way of handling this, but I haven't looked at it yet. I'm happy to include another commit to handle Globalize if need be.