Add Gradle support

# Conflicts:
#	src/main/java/duke/Storage.java
#	src/main/java/duke/TaskList.java
#	src/main/java/duke/Ui.java

# Conflicts:
#	src/main/java/duke/Parser.java
#	src/main/java/duke/TaskList.java

Included some assertions in Parser, Task and Storage class.

These assertions should help to flag out bugs in these classes during
runtime.

Improved on quality of code according to code quality guide. Removed
instances of magic number and revised bad variable naming.

This makes the code easier to read.

Add assertions

# Conflicts:
#	src/main/java/duke/Storage.java

Branch a code quality

Used a HashSet to store task strings for duplicate checking.

Checking for duplicate via a HashSet is faster than checking for
duplicates by iterating through an ArrayList.

Find command used to search using toString method of the tasks.
This will include (by:...) and (at:...) in the search. This will
mean that "find y" will return all deadlines and "find (" will
return all deadlines and events.

Storage used to save only after bye command. Now save after every
modification to task list.
Also updated storage file chick.txt to not store uncheck command
as it is redundant.
Duplicate check was missing in the implementation of removing the
task from duplicate check after deleting the task from task list.

Storage used to save only after bye command. Now save after every
modification to task list.
Also updated storage file chick.txt to not store uncheck command
as it is redundant.
Duplicate check was missing in the implementation of removing the
task from duplicate check after deleting the task from task list.

Changed backslash in userguide to forward slash.