convert_memory_engine

When i use rspec to do a lot of test, each time database is inserted data and truncate table, when tables grows the speed grow down too much. At first i convert the db engine from innodb to memory in db/seed.rb file. After that i write this gem to do the job for more app. Notice: Tables those contain text col can not convert to memory engine, so i convert those colums to string first. So if there are test case use too long string to insert, there would be trouble.