Remove old SBJsonParser and rename SBJsonChunkParser to SBJsonParser

- Updated tests to suit.
- Rename item "return" block.
- Rename the main constructor / initialiser. (Again.)
- Add two more constructor methods for the parser.
Add a bit of documentation to the blocks
Rename classes, constants & enums to add major version number (4)

This is to allow SBJson4 to be used alongside previous versions of itself. This
can be useful now that SBJson is embedded by so many popular libraries.
Additionally, if you're already using it you may not want to replace all usages
in one go; this way you don't have to.
Add documentation for more convenience constructors

Also rename arguments in initialiser to be in line with constructors.
Update README
Generate new version numbers and NEWS entry for 4.0.0-alpha3