Merge pull request #48 from tgrk/master

Implementation of brain.
Merge pull request #49 from 0xAX/Ybot-mnesia-backend

Ybot mnesia backend by @tgrk
rebar.config plugins sub_dir added for erlang/otp plugin compiling. memory plugin some little fixes. ybot_manager.erl added checking erlang/otp plugins applications file finding. ChangeLog.md updated.
checking_new_plugins, checking_new_plugins_timeout, commands_history and history_command_limit_count aren't required parametes now.
comments fixed
return value fixed in ybot_plugins_observer