finished improvements.

main points:
1. Using a hash map as the internal data structure, instead
   of an array and an object, will result in faster processing
   times of operations such as additions, deletions, purging
   for expired tasks, etc. This is crucial if dealing with many
   thousands of tasks.
2. Simpler and faster implementation of the public methods.
3. Separation between public methods and private methods of
   the module.

documentation:
updated both the readme and the internal documentation of the
module.

testing:
1. created unit tests for the public methods.
2. created a will-documented example script 'test/example.js'