Implement <NSCopying> for GTOID.
Add init method to initialize GTOID with a SHA C String and use this in the initWithSHA: method.
Add convenience methods to create GTOID objects.
Fix style issues.
Add unit test for -initWithSHACString:
Fix more style issues.
Add documentation and fix style issues for the convenience initializers.
Add initializers that can return a NSError instance on error.
Fix formatting of documentation comments.
Better error message for invalid SHA string.
Test that an error is returned when GTOID is initialized with an invalid SHA string.
Merge pull request #206 from 5sw/gtoid

GTOID improvements