Merge branch 'master' into interface
Changed classpath and added ANTLR lib

To support natural language parsing of dates from user.
Move toString for DataPair to correct place
Add safety checks in DatePair to guarantee startDate is before endDate
Move isWithinPeriod method from Logic to DatePair/Task.

Note that no test is created yet.
Merge branch 'quality_control'
Hotfix checking in DatePair

Note that setStartDate and setEndDate should NEVER be called manually in code. These two methods are for XML loading ONLY.
Fix a logic bug in DatePair.isWithinPeriod()
Temporary fix for Task.toString()
Fix a possible bug in Task.isWithinPeriod which may be triggered when there are several null values in dateList
Refactor a little bit in LogicTest
Merge branch 'quality_control'
Merge branch 'master' into interface

Conflicts:
	.classpath
Integrated mark command and NLP for date parsing

- Mark is now an acceptable CommandType and parsed.
- Natural language for date is supported for parsing
Merge branch 'master' into interface
Merge branch 'interface'
Update README.md

Provided basic product description, support commands and expected
output format.
Merge branch 'interface'
Fix javadoc in Logic.java
Remove unnecessary try/catch block in Logic.viewTask