Refactored to php-fig/http-message 0.5.0

- Refactors to follow the new interfaces as presented in php-fig/http-message
  0.5.0. Hopefully this will be the last major refactor of the component.
- Tests have not yet been run or refactored.
Tested refactor to psr/http-message 0.5.0

- Refactored tests to follow new objects/interfaces.
Updated to psr/http-message 0.5.1

- setBody() no longer accepts null.
CS fixes

- phpcs does not like aligned operators in argument lists.
Deprecate marshalUri properly

- Added getHeader() static method for doing case-insensitive fetching of
  headers; values are comma-concatenated, allowing for safe lookups and
  consistent return values. This allows removal of marshalUri() in the future,
  and, for the short-term, having it proxy to marshalUriFromServer().
Updated CHANGELOG for 0.7.0
Merge branch 'feature/http-message-0.5.0'

Close #15