Explicit query API

[agateblue]

Implement a more explicit query API, as described in #15.

The PR achieve the following results:

• Implement the explicit query API suggested by @angru (with some changes)
• Keep full backward compatibility with existing query engine using keywords arguments, by converting them internally to explicit queries
• Unittest each query engine
• Add some basic models to make querying using the explicit engine easier
• Split manager, queries, models and exceptions into dedicated files

That means you can query data in lifter using two type of queries. I will document this in another PR.