Make the module compatible with CentOS. It includes :
 * moving all paths / users to a bind::params class
 * define parameters for CentOs
 * remove the bind::debian class, everything is now done in bind::base,
   only parameters change behaviour
 * parametrize all required resources

Spec tests still pass. Still need to add new tests.
Parametrize tests. There is now a spec/defines/parameters.rb which defines the different OS supported, and the list of corresponding variables. _spec files then iterates over the different OS and run tests that are parametrized with the variables if needed.
Add CentOS to the list of supported OS in README
add doc for bind::key