Added Tags table so WeightExercises or AerobicExercises can store multiple tags for the user to track
Added a SiteUser foreign key to Tag to make it easier to get all the tags for a single user
Fixed the repr for Tag object
Made models.py pep8 compliant
Added a RegexValidator to Tags.  Still need to test
Added validator to Tags.  Tags must contain only A-Z, a-z, 0-9, -, _, and must be non-zero length
Merge pull request #104 from CSc436/103AddTagsTable

Added Tags table so WeightExercises or AerobicExercises can store multip...