Added NSJSONSerialization+Misc.
Added NSDictionary.dictionaryWithKeysAndMappedValues.
Have NSDictionary.map return NSMutableArray, since that's more useful.
Have setObject:forKey and removeObjectForKey: return whether they
succeeded.

This means that they don't match the NSUserDefaults interface, but
that's OK.  They're more useful this way.
Improve comment.
Add setObjectFromString:forKey:.

This sets a TBUserDefaults value by parsing the given string.  This
uses the type of the registered setting to guide the parse.

As part of this, pass the type info from the TBUserDefaultsRegisterSettings
macros to be saved in TBUserDefaults.
Add toJSON and registeredSettingsToJSON.

These return JSON-style dictionaries containing the current settings
and the registered settings respectively.