Support subqueries

[uranusjr]

Based on the discussion in #9.

I added an extra layer _query between str and raw to use internally. Statement-based callables and identifier now both return a _query object instead of str. This difference is used in identifier to avoid applying unintended processing to _query objects that represent subqueries.

Now you can do this:

subquery = select('person', select=('last_name',))
print select('person', where={'first_name': paren(subquery)})

And get:

SELECT * FROM "person" WHERE "first_name" = (SELECT "last_name" FROM "person")

The above test has been added to build.select.

[moskytw]

I did some changes basing on your code:

1. The mosql.build is deprecated, so I removed the doctests. I will re-added them in query.rst. (see 9141715)
2. Actually, supporting subquery doesn't need so much code, so I removed some. (see 679f758)
3. Finally, I use another approach instead of _query. It is simpler but slower I think, so I am not sure it is better or not. (see 36c1ae9)

Maybe you will want to make sure the functionality is still correct.