Allows for custom headers to be set in message

* Adds setHeaders, addHeader, getHeader to {MCO,MC}MessageHeader
* Exposes MCHashMap iterator for more effecient iteration
Fixes for PR feedback

* Reverts MCHashMap changes, uses mc_foreachdictionaryKeyAndValue
* Adds getter for mExtraHeaders to MessageHeader
* Adds removeHeader
* Tweaks method names
Whitespace cleanup
Fixes for PR feedback

* headerForName -> headerValueForName
* setExtraHeaders made private, removed extraHeaders
* added removeHeaderForName
Adds NULL check and style cleanup

* Adds check for mExtraHeaders == NULL to all allExtraHeadersNames()
* Makes mExtraHeaders == NULL checks more consistent
* Cleans up code formatting to be more consistent
Merge pull request #161 from thusfresh/CustomHeaders

Allows for custom headers to be set for message builder.