Added threading feature.

[gautamkhanapuri]

When working on a project at my workplace, I found this library very useful.

I have added the threading feature. The user needs to create an instance of Subset of Consumer class. He can then call the .start_consumer() instance method which will intrinsically create a thread if the threaded variable is set to True . The user has the option to create a daemon or non-daemon thread. The user need not worry about joining or stopping the thread manually at the time of program exit. It is done automatically. It can be done manually if they wish to. The changes have passed all the tests. Let me know if I have to add any tests. I will change the documentation as soon as you say the changes are ok. If there are any changes you would like me to make, please let me know.