Adding support for the decode_content request option

- MessageFactory and adapters now handle decode_content
- server.js now requires base64 encoded data in the "body" JSON parameter.
- Closes #764
Added the ability to throw exceptions immediately.

Closes #760
Adding more information on parallel adapter error handling
Fixing typo
Clarifying error handling behavior
Merge pull request #772 from guzzle/version-4-2

Version 4.2