Update filters.rb

Only apply the filter onto the result if it's valid, a badly coded filter block could return anything which would reset result.

It may even be worthwhile checking if filter_result is a ActiveRecord::Relation?
Merge pull request #63 from markedmondson/patch-1

Update filters.rb
Add test for returning nil from filter
Version bump to 0.8.0
Regenerate gemspec for version 0.8.0
Generator doc
Remove dev junk
Array driver
Merge branch 'master' of git://github.com/bogdan/datagrid