Improving documentation on JSON serialization

There was some confusion that MTLJSONAdapter should be used instead of
+[MTLModel modelWithDictionary:]. See #94.
Reformatting the README
Adding code examples to README
Fixing possive of superclass
More README improvements
Improving style
Merge branch 'master' into improving-docs
Fixing property declarations

No clue what was going on there
Structuring +JSONKeyPathsByPropertyKey explanation
Removing mention of date formatter sharing
Moving up MTLJSONAdapter section
Fixing typo
Removing XYEntity form XYUser example
Improving documentation for +JSONTransformerForKey:
Improving documentation for +classForParsingJSONDictionary:
Fixing XYMessage superclass
Show -mtl_dictionaryByAddingEntriesFromDictionary
Minor README tweaks
Merge pull request #128 from github/improving-docs

Improving documentation on JSON serialization