Tab change handling and other updates.

Run Heartbleed check when user changes tabs - first checks cached sites
and if no cached entry exists then run remote check. Close open
notifications when user changes tab. Support short term caching for
sites that returned an error during check. Added constants file.
Merge pull request #28 from olivitek/master

Tab change handling and other updates.
Thanks @olivitek we will hopefull be able to release this in next sub release!

Closes #29