Fix bug where @and removes existing wheres

Example:
array(
	'foo' => 1,
	'@and' => array(
		'bar' => 2,
		'baz' => 3
	)
)
Would give the query "bar = ? AND baz = ?"
 with params = [1, 2, 3]

Expected result would be "foo = ? AND ( bar = ? and baz = ? ) "
with params = [1, 2, 3].
Merge pull request #21 from torandi/and_subquery_bugfix

Fix bug where @and removes existing wheres
Merge remote-tracking branch 'edruid/master'