webapi format symbol and post data validation, specs, refactor

[agramian]

• webapi makes sure the request contains a valid format symbol (.json or .xml) and returns appropriate error message if not
• webapi catches exception if invalid post data is given and cannot be parsed, returns error message
• added specs to invalid_api_spec to verify the new functionality
• for all xml responses, renamed root element from ‘hash’ to ‘response’
• refactored ‘request_params’ to ‘request_data’