Support for custom number of successful probing requests before moving a destination from 'inactive' to 'active' state

Hi, I've added support for custom number of successful probing requests before moving a destination from 'inactive' to 'active' state.
The feature is the same of probing_threshold but it counts the number of successful ping before moving back a destination from 'inactive' to 'active' state.
Hope it could be useful to someone else. I've also edited README file inserting the new parameter.
I've corrected somewhere in the code some misspelled words and changed some variables (I used the same counter for failure and successful requests, these features are mutually exclusive).

Please note: I've developed and tested this feature over release 4.1.6