#340 - [api] Add support for 'pre-encoded keys'

[rbygrave]

Refer to #340

Create a JsonGenerator.Key

JsonProvider provider = ...;

JsonGenerator.Key firstNameKey = provider.createGeneratorKey("firstName");

Use a JsonGenerator.Key

JsonGenerator generator = ...;

// we can use the key rather than just a string key for performance reasons
// ... it's already escaped 
generator.writeKey(firstNameKey).write("foo");

// rather than string key 
generator.writeKey("lastName").write("foo");