Implement "AS" syntax and some standard SQL functions

See `function.py` for implemented standard functions.

I'm not sure where the docs should go, so I didn't add any, sorry. :p
Might want to hint the user in the docs that they can `import mosql.function as f` to use `f.sum` or similar. Don't `from mosql.function import *` unless you know want you're doing because it will override the built-in `min`, `max` and `sum`.