Added support for filtering issues by severity

Changed sproc_issue_getAll to include a severity parameter
Changed library and added required API routes
Added backbone routes and functionality, along with defaults object for
params
Added dropdown to filter issues by severity

* Added dropdown to specify severity. Using a kinda convoluted way of
using .issue-opt values to construct a route. There may be a better way?
* Was going to use _.extend, but it overrides properties even if the
source's property value is undefined. Using $.extend instead.
* Changed how option specifications work when changing dropdown values
for filtering issues.
* Added a ``params`` object for loadIssues, which overrides defaults