Rename OpenVsResolved to indicate it is a graph

Add an Issue by Priority graph.
Create base class for graphs

Refactor the two graph classes to use a common base class to allow for a
common look and feel and eliminate duplication.
Add a title to each chart
Clean up of some of the graphing and use cases for the IssuePriorityGraph
Updates to OpenVsResolvedGraph unit tests
Merge branch 'master' of https://github.com/benhejl/SWENG500_Team1