Use optional AS keyword in alias clauses

Modified engine to use AS keyword. Also fixed affected unit tests
which cover aliases for fields, tables, and subselects.
Use <> instead of != in relational comparisons

Using this form of comparison allows Microsoft Access queries
that test for inequality to use the normal predicate syntax
instead of calling the 'raw' function. I believe <> is valid
generic SQL and still works with all the other vendors.
Merge pull request #117 from RingMan/not-equal

Use <> instead of != in relational comparisons
Merge pull request #116 from RingMan/use-as-in-alias

Use optional AS keyword in alias clauses