Support for hiding output.

This commit adds extra functionality in the notebook and at the
paragraph level to selectively hide/show the output.  This is useful
when the output is of little importance, for example, a bunch of imports
or declarations for which the Scala REPL output is redundant.
Fix javascript typo
Added paragraph button for show/hide output.  Removed this toggle from paragraph drop down
Merge pull request #172 from coolfrood/topic/support-for-hiding-output

Support for hiding output.