adding some toString tests and refactoring parameterNode creation logic
MySQL does not do date UTCString, but ISOString format works
Merge pull request #107 from yc662/toStringDateObject

Date object toString using toUTCString, refactor duplicate code for ParameterNode, added a set of toString tests
Merge pull request #109 from kevinoid/for-upstream/test-js-only

Only test .js files in test/dialects
Split Unary into PrefixUnary and PostfixUnary

Separate unary nodes into separate prefix- and postfix- unary nodes in
preparation for adding prefix unary operators to the supported grammar.

Signed-off-by: Kevin Locke <klocke@quantpost.com>
Add support for EXISTS and NOT EXISTS expressions

Support checking the existence of subqueries using the SQL EXISTS and
NOT EXISTS expressions by adding `exists` and `notExists` methods to the
`Query` prototype.

This syntax is backwards compared to the SQL construction (which is
EXISTS [...] rather than [...] EXISTS) but seems more reasonable than a
putting `exists` in a shared scope.

Note that `notExists` could be implemented just as well with a unary
logical negation around the unary EXISTS, but since this is not done
elsewhere, it seemed preferable to stick with the existing style of
combining the negations with their positive counterparts.

Signed-off-by: Kevin Locke <klocke@quantpost.com>