Simple slicing implementation

A (kind of) working implementation of slicing. The problem here is that neither MySQL and SQLite supports OFFSET without LIMIT, which is troublesome. Some choices here:

1. Set a default for LIMIT if necessary. This is the default implementation (using sys.maxsize for a "very large integer"), but is not without problems. One of those is that sys.maxsize may not be enough -- MySQL supports up to 2^64 - 1, for example, but sys.maxsize is only 2^63 - 1 (because it's signed). There might be problems in cases that sys.maxsize is too large and causes overflow on the database side.

2. Let the user figure a way out. MoSQL generates SQL fine with or without LIMIT, so we can just let the query die if the user does the wrong thing. Kind of harsh in my opinion though.

3. Do some crazy database detection at runtime. Django supports only limited database versions, so we actually can exhaust all possible cases, and provide sensible defaults as needed. But might be a bit ugly? Maybe we can take a peek into Django's implementation to get an idea.

Need to sit on this for a while to decide.
Add a test for param keys
Use Django's implementation to resolve LIMIT
Merge branch 'slicing'

Fix #5