Added call to mysql_library_init during initialization of the gem

  This call must be performed before trying to call mysql_init from multiple threads
  Reference: http://dev.mysql.com/doc/refman/5.1/en/mysql-init.html
  Minimal reproduction of the problem if mysql_library_init is not called

    require 'mysql2'

    def connect
      Mysql2::Client.new()
    end

    threads = [0,1].map {
      Thread.new { connect }
    }
    threads.map(&:join)
    puts "OK!"
Merge pull request #530 from michaelkruglos/master

Added call to mysql_library_init during initialization of the gem