SQL Datashape of Decimal not Supported

I tried using blaze on a mysql database and got:

```
No SQL-datashape match for type DECIMAL(18, 5)
...
No SQL-datashape match for type DECIMAL(11, 5)
```

I'm using mysqldb, if that matters. It seems that there was some discussion [in the forums](https://groups.google.com/a/continuum.io/forum/#!searchin/blaze-dev/decimal/blaze-dev/2B5aWGKm-xU/ep46UCcqDekJ) and ContinuumIO/datashape#118.
