record the points_available to the db

This will allow the value to be customized
Add a method to get the percent grade for the metric
Calculate grades with the new points_available field
Allow customizing the points_available in the example metric
Update tests to reflect changes
Add a new test to verify that customizing the points_available is working
Remove debug statement
Fix failing test
Add a test for getPercentGrade() and fix rounding error
Use 'points_available' to match the database name
Add a comment for the points_available column
Merge pull request #28 from mfairchild365/scan

Allow customizing the total points available for a metric