emysql_query and emysql_saver

this pull request includes three new modules:

* emysql_pool.erl: gen_server which maintains pools from OTP application easily
* emysql_query.erl: which contains find, find_first and find_each functions, and these functions wrap sql queries and record together
* emysql_saver.erl: which makes saving or updating record in another convenient way.

2414e7c commit is mnesia style transaction you may not interested in.