[Feature] Added new feature to allow union

Feature added to select query builder.

squel.select().field('field1').from('table1').union(squel.select().field('field1').from('table2'))

Fixes have also been applied to the mocha tests.  useFlavour was mixing the versions of select, update,... so tests would fail.  Now the flavour based tests force a reload of the squel module before each test.  Non-flavour based tests have been separated into their own test grouping and run first to prevent flavours from invalidating the test results.

Updates:
> A new UnionBlock has been created and added to the end of the select query builder.  Must be at the end to ensure the union statement occurs at the end of the first query.
> toParam() for numbered parameters has been catered for by adding a new option that is passed to the sub-select query builder.  The nestedBuilder option is used internal to the builder to only update the query parameters with numbered parameters if it is the parent query builder.
> Test cases added to select.test.coffee and postgres.test.coffee for union and toParam calls.
[BugFix/toParam] Add support for toParam to JoinBlock and AbstractTableBlock