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.
Instead of returning `false` here, I guess we should return `null`. Also, php-fig expressly doesn't specify commenting style requirements, but I prefer one-line comments like this to be on their own line before the line being referred to.