Hold onto current slot context for timer
Hold onto current slot context for download
Hold onto slot context for animation

This includes a new helper to create a proc bound to execute in a given slot.
Update Shoes::Timer to use slot's block binding
Update Shoes::Download to use slot's block binding
Extract method and use a local in download
Rename to the slightly sleaker create_bound_block
Spec for creating bound blocks on slots

Minor cleanup of unnecessary self as well.
More thorough specing around bound blocks
Merge pull request #962 from shoes/slot-context

Capture slot context for timer, download and animation

* Changelog: improvement