Algorithm Dijkstra, export to GEXF

1. New classes PWVocab (vocabulary LANG_CODE), PWRelatedWords (related
words LANG_CODE)
2. Additional table pw_short_path_LANG_CODE for shortest path between
words, new class PWShortPath for service this table
3. New class PWGEXF for export the graph of related words in GEXF
4. short_path_search.php - search shortest paths between given words
5. new methods DijkstraAlgorithmByArray and DijkstraAlgorithmByDB for
searching shortest path between words by writing intermediate paths to
DB or arrays.