Construct ObjectMapper once per RestInvocationHandler

This pull request is motivated by http://wiki.fasterxml.com/JacksonBestPracticesPerformance

Currently:
- a new instance of ObjectMapper is constructed for writing each request invocation.
- the Jackson configuration only applies to the ObjectMapper used to read

This pull request changes things as follows:
- RestInvocationHandler is responsible for constructing an ObjectMapper and configuring it
- the same ObjectMapper is passed into HttpTemplate & RestInvocation

If you have concerns about this PR, please let me know what they are and I will try to address them. Thanks!