MA-35 MOTECH Pubsubhubbub implementation - Hub module

This is a merge of the dev/hub branch.

This commit:
* Adds the motech-platform-hub module
* Adds http-agent module required by the hub module (MA-340)
* Adds support for executeSyncWithReturnType to return the response value
* Adds support for retryCount and retryInterval added for executeSyncWithReturnType in http-agent
* Adds support for GET method in http-agent

Functional and Technical documentation for hub module: https://docs.google.com/document/d/1nk8roTHc6jN_cjcrOLAXaI_rvr36RYxdl0PULKozdiQ/edit?usp=sharing
Test case documentation for hub module: https://docs.google.com/spreadsheets/d/15mNTBoR2Vdao-tP8Mg1s30LFMMuj3_iU9qccBe6dcVo/edit?usp=sharing

Change-Id: Id856ba7d2bbfee9c2e00f8dce0f935af38c013ac