Added functionality to allow an ignore class

You can add an ignore to the options that allows the filter to ignore
children that have that class. The ignore defaults to filter-ignore. To
support this changed to use the jQuery.text() function.
I implemented this to allow the filter to be used on a table and still restrict certain table elements from being included in the filter. There may well be a better way to do this but this gave me what I wanted and I thought it might be useful. This is my first Git pull request so I probably did it wrong and for that I am sorry.