Add some stats functions

[loiluu]

Some interesting statistics we may wish to collect

• Number of miners
• Computing total hash rate in the pool

(Please feel free to add more)

[yaronvel]

I will work on it.
Note that most statstic will be gathered by a javascript client/webpage.
I will added minimal needed support in the contract (events) and make a javascript code that calculates it.
Once we start working on front end, we can make it less ugly.