Initial portion of unit-tests

Hi Geremy,

Could you review changes?
There are 69 tests.
During integration with master repository I've reached issue with KVO in init method(PNReachability) and unit-tests. After long investigation and analyzing I decided to replace observing private variable with standard approach of override setter. On this part I've changed on internal implementation of PNReachability and any changes for interface. Also I should notice that after this changes in general the code of PNReachability is more safety to work with.

Regards,
Vadim