modify to support multiple grammars, add fmpxmllayout grammar

The approach taken was to convert the execute() method to route based on grammar and to move all of the parsing into grammar specific parsing methods. Also added an array property to validate that a supported grammar has been specified. fmxmlresult is the default, to support backward compatibility.
Also speaking of tests, your current build passes all the existing unit tests after I delete the two that are now obsolete. You might as well include the deletion of the irrelevant tests in your PR:

https://github.com/soliantconsulting/SimpleFM/blob/master/tests/SoliantTest/SimpleFM/AdapterTest.php#L200

https://github.com/soliantconsulting/SimpleFM/blob/master/tests/SoliantTest/SimpleFM/AdapterTest.php#L211