First stab at adding MetadataManager

This is NOT ready to merge yet. I'll be adding implementation for the functions in the next couple of days.
Formatting request: these method definitions are hard to read in the IDE, given their length.  Can we add some newlines to them?  E.g.

        - (void)loadSmartScopeObjectTypes:(SFDataCachePolicy)cachePolicy
                  refreshCacheIfOlderThan:(NSTimeInterval)refreshCacheIfOlderThan
                          completionBlock:(void(^)(NSArray *results, BOOL isDataFromCache))completionBlock
                                    error:(void(^)(NSError *error))errorBlock;