dynamic backend managing; split buffer display improvement; phrase replacing algorithm improvement

V0.5 Changelog

Changed

• Improved syntax of =part of speech= in split synonym display
• Improved split window display for synonym candidates.
• Implemented dynamic backend-managing mechanism. Now adding new backends
  is as simple as dropping the backend file into folder
  =./autoload/thesaurus_query/backends=

Fixed

• Fix multiple bugs encountered when querying for words/phrases surrounded
  by non-space symbols or contain non-space symbols.
• String =zero length field in format= issue with Python2.6.