add keyword recognition for "FULL OUTER JOIN" and "CROSS JOIN"
add FULL to common keywords (frequently used in OUTER JOIN definitions)
extend regex to allow recognition of 'NATURAL JOIN'
tighten up JOIN regex

ensure the regex does not match against invalid permutations such as 'LEFT OUTER CROSS JOIN', since CROSS/NATURAL keywords were added
Merge pull request #112 from alexander-beedie/master

extended JOIN token recognition/lexing (eg: FULL OUTER JOIN, NATURAL JOIN)