Added support to run MySQL on RAM

I've added support to mount the MySQL datadir into an tmpfs partition. This is useful to run test suites against MySQL faster.

Here is how it works:

`MYSQL_RAM=1` turns on the feature.
`MYSQL_RAM_SIZE` defines the amount of RAM allowed.

Note: this requires container extended privileges (-privileged), otherwise an `Permission denied` error occurs.

Example:
```
docker run -e MYSQL_RAM=1 -e MYSQL_RAM_SIZE=256 --privileged -d --name mysql-ram orchardup/mysql
```