Added `count()` and `countTrashed()`
Bumped version to 0.2.0-alpha.0.
Added more static functions.
Added `model#trash()` and `model#restore()`.
Bumped version to 0.2.0-alpha.1.
Changed API: `model#trashBy()` is removed, and replaced by `Query.by()`;
`model.trash()` and `model.restore()` now return `Query` to more closed matched
with the native mongoose APIs.
Bumped version to 0.2.0-alpha.2.