Add the ability to compare BoxModels to each other

This commit adds a compare:usingComparator: method to BoxModel as well
as several comparators that order models based on their properties.

Included comparators:

* type and id
* name
* size
* created and modified dates
* SHA1
Merge pull request #20 from box/feature/model-comparators

Add the ability to compare BoxModels to each other