Simple tests for sub-set projections.
Support subsets fields filtering.

Rewrote the returned closure. Still don't support '$' and '$slice' operators.
Improve documentation on fields.

- You can mix incl/excl as long as it is `_id`
- Note unsupported `$` and `$slice` operators of minimongo
- Give an advanced example with reference to mongodb docs.
Whitespace
Add nested-nested test.

Taken from Glasser's old code-review.
Fields with subsets passes the test.
Same test but for blacklisting
Fix docs. $slice is not related to fields projection.
Put a.b.c and a.b validation into general case.