Complete implementation of SPUser's 'customObject' stub methods

[converted2mac]

Issue to track the progress of the methods which currently sit in the SPUser class. Excited to be able to attach some custom user info to the actual user account, rather than trying to implement something that syncs as a core data object.

-(void)setCustomObject:(id)object forKey:(NSString *)key;
-(id)getCustomObjectForKey:(NSString *)key;

[mikejohnstn]

Agreed, this is a pretty important capability. @fredrocious

[mikejohnstn]

We're still working on the backend for this. Moving to v0.8.0 release, which will extend SPUser quite a bit.

[jleandroperez]

Closing in favor of Issue #442