added javadoc for recurrence, taskID, tasktype, time, title
added javadoc for classes

Complete, DeleteOption, Description, Keyword, Location
added javadoc for CommandType, Option, ParameterList
added testcases

added more assertion checks for
- CommandTypeTest
- CompleteTest
- DescriptionTest

Added new testfile:
- DeleteOptionTest
updated testcases

added extra assertion tests for:
- TaskIDTest
- ParameterListTest
- KeywordTest
edited javadoc
updated testcases and added assertion tests