SQL2 Parser literal parsing

[dbu]

in #99 we noted that parseLiteral stumbles over its call to $this->scanner->getPreviousDelimiter() when there is a . in the literal. we need tests about this and probably have to fix something. once it works fine, we can cleanup the parseCastLiteral code added in #99 to call parseUncastLiteral

[dbu]

we meanwhile have lots of phpcr-api-tests that parse literals with ., e.g. in the Sql2TestQueries. they seem to work fine now.