This uses the mysql driver library's capability to use `SET` to set the system
variables that prefixdb previously was.
Unfortunately, the library doesn't sort the params when making the string, so we
have to do a little munging to TestNewDbMap.
Ran it in a checkout of the repo since godeps now doesn't include the test files (which is great!).
```
MYSQL_TEST_ADDR=127.0.0.1:3306 go test .
ok github.com/go-sql-driver/mysql 46.099s
```
Jeff Hodges