Add error handling by configuring error procs on `Deas::Server`

This adds error handling by allowing setting error procs on
the `Deas::Server`. These procs are triggered anytime Sinatra
would trigger it's `error` or `not_found` handlers. They are
passed the exception and evaluated in Sinatra's context, which
allows doing anything possible with Sinatra. This allows apps to
setup error handling easily with the all the features of
Sinatra.

Closes #21
Merge pull request #41 from redding/jcr-error-handling

Add error handling by configuring error procs on `Deas::Server`
change the default `sessions` setting to be `false`

This means that no session cookie is used by default.  This is the
responsibility of the developer and should not be enabled without
them doing it.

This helps with security as no cookies are used without the developer
knowing about it.