Implemented the Category function into the parser
Added in validity check for dates.
Change add function
-Category is able to be added anywhere in a string
Added test case for new changes to add function
Change edit function
-Category is able to be put anywhere in the string
Added test case for the changes for edit function
Implement completed command
Implemented Category parser class
Moved CommandParser as static and requires ToDoStore object
Fixes updating todo to completed
Merge pull request #46 from cs2103aug2014-t17-3j/Parser

Parser Implementations.
- Added Category parsing capabilities.
- Added marking todo as Completed.
- Moved Parser as static functions.
- More test cases for Jenkins.