remove `partial` method from view handlers and runners

This isn't a universal capability across template engines so it
shouldn't be available at the view handler level.  Template engines
should be the ones to provide this function if they choose to.

This was really just a macro for a special `render` call anyway so
we aren't really losing any capabilities.

In a future PR, the `partial` method and `Partial` template subclass
will need to be removed from "general" deas and into an erb template
engine gem (once the API for template engines is ready).  Again,
this isn't a global template engine capability so it shouldn't be
exposed as one.

This is all prep for building a template rendering API and not
relying on Tilt/Sinatra.
Merge pull request #129 from redding/kr-rm-partial

remove `partial` method from view handlers and runners
runners: have base runner take args and set its readers

This reworks how the runners set the necessary base runner attributes.
This makes for a more uniform API across the runners.

This is also prep for adding in a `DeasRunner` that takes args in
this manner.  The `DeasRunner` is the future counterpart to the
test runner and will be added to transition off of using sinatra
at the runner level.  The sinatra runner will subclass the deas
runner and then we can start porting logic out of the sinatra
runner and into the deas runner.

This is the first step towards that setup.
Merge pull request #130 from redding/kr-sinatra-runner-args

runners: have base runner take args and set its readers
runners: remove `app_settings` reader from the runners

This is prep for moving away from the sinatra runner.  This is a
big fat coupling between sinatra and anything that uses the runner.
Not what we want going forward and can't be supported moving away
from Sinatra.