Fix bug at GoogleController modifyTask

Under the method modifyTask, changed from ConcurrentDelete callable to
ConcurrentModify callable.
Added comments to important methods in GoogleController

This commit adds comments to important methods.

It also re-orders the method appearance in the java file.
It also refactors the initialization method into local and remote so
that local components can be initialized as soon as a new
GoogleController object is created.
Removed redundant imports.

Callable and DateTime are removed as GoogleController no longer needs
them. These are handled by the deeper components that GoogleController
manages.
Fix bug in ConcurrentModify and ConcurrentAdd

The bug occurs when chirptask.storage.Task.getDate is called even if
the task is a floating task. A floating task will throw a null pointer
exception when .getDate is called.