Clean up some exception class names
Add Task#find_by_id & tests, plus more Task tests

Task#find_by_id can be used to request a task directly - this is handy,
for example, when generating a Tasks index page with links to Task show
pages.  Also introduced more robust specs to test bounds for other Task
methods.
Whitespace
Worker join tests too fragile; replaced with stubs

These were spiraling out of control like the Tazmanian devil up in here.
Factor out redis and sequel (for external gems)

Requiring ruote-redis and ruote-sequel here created a large dependency
stack that was unneeded if you weren't using both adapters.  Both have
been removed and are being set up as separate gems.  If either of these
two adapters are defined (e.g. if they were put in the Gemfile), they
will be automatically loaded.
Better Rspec result formatting
Bump version number