fix for "undefined" showing up in the dashboard on the TIME SPENT widget.  This commit only fixes it for the US English translation, though.  To fix it for the other translations, a string named "common.year.abrv" needs to be added to the other language dashboard_*.properties files.

Bug references:
https://github.com/Countly/countly-server/issues/88
http://support.count.ly/discussions/problems/282-some-of-ui-elements-are-undefined

This commit also contains a fix for a separate minor misspelling error in the US English translation.
fix for issue #98: https://github.com/Countly/countly-server/issues/98 - ignore session_duration values <= 0

additionally, I made the obscure check for NaN more explicit; also made sure the callback was called in processSessionDuration even if a bad session_duration value was encountered
additional fixes for https://github.com/Countly/countly-server/issues/98
Closes #81
respond with 400 if events or metrics JSON data is not properly formatted.
/i: always response 200 to client
/o: normal response 400 to browser

Conflicts:
	api/api.js