Allow single quote in style attribute values.

E.g. "font-family: 'Tahoma'" should not be removed.
Remove quotes around addressee name in case they are encoded.

Some clients encode quotes with name using base64/quoted-printable
encoding. Quotes were removed only for unencoded strings. Now also
encoded names are handled this way.
So, recipient/sender strings are displayed without quotes around
and e.g. saving to addressbook does not save these redundant quotes.
Fix confusing intersect/filter methods naming/behaviour.

Removed rcube_result_index::intersect() method.
Changed rcube_result_index::filter() to filter in the same way as
rcube_result_thread::filter(), which means it actually does array_intersect().
Added tests scripts for rcube_result_index class.