Refs #185, #175, #168, #60 - Tests and fixes for cast and output formatters

• All primitives now permit null values.
• Output of a single string is now valid JSON - i.e. quoted.
• Text types are not accidentally cast and are treated as strings.
• Utilising soapbox/laravel-formatter for conversions between json/array/xml.
• Created a special class for json/array/html conversions that that there is no data loss from a HTML doc.
• Cast processor updated to correctly handle the edge cases.
• DataContainer refactored.
  • automatic casting on:
    • constructor() if data_type declared.
    • setData() if data_type declared.
    • setType().
• Testing for empty to account for int (0) and boolean (false).
• Full tests for Cast processor and all permutations of output data and Accept header.