Added slow_query_log and slow_query_log_file variables.

This pull request adds the variables slow_query_log and slow_query_log_file to the mysql::config class.

MySQL 5.1:
* [slow_query_log](https://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html#sysvar_slow_query_log)
* [slow_query_log_file](https://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html#sysvar_slow_query_log_file)

MySQL 5.5:
* [slow_query_log](https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_slow_query_log)
* [slow_query_log_file](https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_slow_query_log_file)

MySQL 5.6:
* [slow_query_log](https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_slow_query_log)
* [slow_query_log_file](https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_slow_query_log_file)

Tests are green:

```
$ bundle exec rake spec
...
Finished in 8.58 seconds
162 examples, 0 failures
```