Refactored HttpClient to send a raw request body instead of only JSON encoded parameters.

This is needed to support Asset creation where the request body must be the content of the asset.

This involves the following changes
- Updated HttpClient methods to accept an untyped body instead of an array of parameters
- Moved JSON encoding to AbstractApi and updated documentation accordingly
- Updated test cases
Add support for asset creation

Added coresponding unit test for PHP 5.3.4 or newer. SSL extensions are not fully working for 5.3.3. See https://github.com/travis-ci/travis-ci/issues/1385.

Updated documentation to show that Asset creation is now supported.