[Tracker] Bug 509607: Make search support logical operators besides 'And'

[Carlos Garnacho <carlos imendio com>]

Hi!,

I made a patch to build an expression tree from search expressions like
"foo and bar or baz" and compose results based on the constructed tree.
The "And" operator takes precedence on the "Or" one, so that search
string would result in:

   And
  /   \
Foo   Or
     /  \
    Bar Baz

The patch is attached to bug #509607, any testing is welcome. I think
the only issue left is translation of logical operators, but shouldn't
be a hard issue, probably we should just add a comment for translators
to tell them what are these strings about.

Regards,
   Carlos