Generic TCP/UDP checker

Thanks to @chesleybrown for adding support for a generic tcp/udp checker in #47!

Minor bugfix release

Fixed state read race reported in #45; fixed an incorrectly placed defer resp.Body.Close().

Introduction of failure/recovery callbacks

Thanks to @schigh for contributing failure/recovery callback functionality!

Failed state recovery bugfix/update

Another minor release that allows the global failed state to recover after a previously failed (and fatal) check recovers.

SQL checker

@schigh contributed an awesome, extensible SQL checker - nicely done!

Minor bugfix release

• Fixed a goroutine leak after .Stop() is called
• States now actually get reset after .Stop() is called
• Minor test cov increase

Initial release

This is the initial release!