Made DatabaseEngine singleton method
Made login use asynctask
Use getInstance instead of new object on heap
Optimizing imports
Major code reformatting
Update username when resuming MainActivity

Earlier when returning from manage user activity the username was not getting
updated even if user changed data there, possibly deleted all user there
Added back stack functionality to EditFragment

	Earlier on pressing back button from EditFragment it
	used to return to MainActivity
	Now it returns to the previous state of the same Activity
	i.e. only closes the EditFragment not the parent activity
Cancel button of EditFragment is functional
FragmentEdit closes on updating a record
Added a few toasts

	Toast messages on:
		update
		cancel
Home screen modified

	Removed:
		TextBoxes - username and password
		Buttons - save and cancel
	Added:
		Spinner for usernames
Changed ManageUser activity screen

	Choosing ManageUser from action bar now shows
	3 options
		ADD
		UPDATE
		DELETE
Update button is functional

	Update button in ManageUser calls EditFragment
Changed Update button name to Save
Changes in FragmentEdit

	*Made changes in FragmentEdit to accomodate it for both
	 add and update option
	 -if add_update = 1 -> add option
	 -if add_update = 2 -> update option
	*add is yet not functional
Added onClick event for Edit button
Major changes to reach FragmentEdit

	*Add and Edit button both need to access FragmentEdit
 	 from ManageUser activity through UserDatabase
	*add_update=true/false value differentiaties between add and
	 update
	*In additon, removed Delete button from UserDatabase's
	 activity
Add button is now functional

	Made new insert function in DatabaseEngine
	Made a few extra checks than previous implementation
	Redundant records will not be added
Removed unwanted comments
Disable/Enable spinner

	Spinner enabled and disabled on switching
	between edit fragment and userdatabase activity