Implement getPriority and enum for prioirty
Changes to these files:
- implement the edit priority function in todoEdit
- modify the priority in ToDo model
Modified Edit function and ToDo model
- cast String to Priority in edit function before passing into the ToDo Model
- modify setter to a proper setter that takes in Priority variable
Changes to these files:
- Change function name in Category Parser and help to parse priority (need to rename the parser)
- Change add function so to add with priority
Fix Bug in add function and modified edit function
Changes to these files
- ToDoAdd File: Changed class name for CategoryParser to CategoryPriorityParser
- added new test cases in AddFunctionTest to test the new implementatin in CategoryPriorityParser
- Modify CategoryPriorityParser to cater for bug such as +Gift+High without spacing in between
Changes to these files
- Modify add function to remove priority such as  HiGh/Medium/meDiuM/LoW/lOw etc.
- Change Class name for CategoryParser in ToDoSearch
- removed unwanted codes in CategoryPriorityParser
- Add test cases for ToDoAdd and ToDoEdit
Minor source format.
Merge pull request #96 from cs2103aug2014-t17-3j/Issue_91,_92,_93_-_Priority_Level

Issue #91, #92, #93 - Priority level