oc-calendar: Retrieve properly event description created on SOGo

As it is stored as plain text without triming string.

The correct comparison here is NSNotFound as stated in Apple documentation.
oc-rtf: Use what RTF Spec 1.9.1 for control words

This reverts 7581c0a.
oc-rtf: Fix current position to match where _bytes pointer is

This avoids to have extra unknown bytes at the end of the RTF document
as _len would have been reached but _current_pos had incorrect less value.
oc-rtf: Be more resilient to spaces while parsing font tables

This avoids to crash on this example:

  \f0\fibi \fcharset0
   -------^

Or:

  \f0 \fibi\fcharset0
   --^

Take into account that I found lots of RTF documents with this format:

  \f0\fbidi \froman\fcharset0
  \f0\froman\fprq2 \fcharset0

Which are not unsupported by this handler.
Merge pull request #41 from Zentyal/ejhernandez/fix-event-description

Event description is now correctly set and get from Outlook