Refactor project structure

[ncopiy]

1. define dirs: backend, fields, searchset, mapping, parser, searchhelper
   includes all sources
2. use imports in django/elastic/base dirs
3. use deprecation warnings on those imports
4. fix setup
5. fix docs
6. fix tests

• TAG 0.1.17

1. include searchhelper into searchset by deafault
2. use deprecation warnings

• TAG 0.1.18

1. remove deprecation warnings from second refactoring wave

• TAG 0.2.0