INT-3122 Add JsonPropertyAccessor

Requires Jackson2

Bump Jackson2 version

Remove write support

Add String support

Polishing

Initialize objectMapper field; assert ObjectMapper is not null.

Newlines at ends of files.
INT-3122 Polishing - Fix Deprecated Jackson2 Call

createJsonParser -> createParser

Also remove some unused imports in MapMessageConverter
INT-3119 Add Documentation Re SpEL Customization

- Custom PropertyAccessors
- Custom Functions

Polishing - PR Comments