Use a hash to calculate initial splay

Calculates an MD5 hash of client_name:check_name and uses this value to
determine when to perform an initial standalone check after a sensu start.

Fixes #795.
[keepalive_output] time since last keepalive and the threshold
Merge pull request #829 from portertech/feature/keepalive_output

cleaned up keepalive check output
Merge pull request #810 from hashbrowncipher/hashed_initial_splay

Use a hash to calculate initial splay