sortAvailable option.

Defaults to true. When false the sort ascending and descending menu
items are not available. Set to false when using slick grid's multi
column sorting.
Add sortAvailable option to README.
Add ability to have unfiltered columns.
Make label css more specific.
Multi-column sorting example.
Search within filtered rows.
Stop propagation of header click.
Example 3 - show status on search as well as on filter.
Merge remote-tracking branch 'remotes/jcsjcs/master'

Conflicts:
	ext.headerfilter.js