Search improvements in api and client.

Added new commands : search_tag, search_tags, search_user, search_user_liked, search_options.
Number of results per page and sorting options can now be changed via search_options command.
Current results page and total results page are now displayed.
Empty line after a search go to next results page.
Refactored search_mix method in api.
Added new commands description in README.
Code refactoring and various fixes.

Add <set> command as replacement for <autologin> and <search_options>
Fix authentification problem.

(un)like, (un)favorite, were not working anymore.
Fixed `obtain_user_token` method : user token is now passed in session headers.
Merge pull request #33 from octogene/develop

Search improvements
Simplified `show_next_page` logic, fixed common prefix bug
Renamed `search_tag` to `search_tags`
Show help for all commands when no argument is given
Made tag search in api.py more robust
Simplified logic in `do_set`