precision and scale not supported for DECIMAL type

The [current implementation of `DECIMAL` (/`NUMERIC`)](https://github.com/dropbox/PyHive/blob/master/pyhive/sqlalchemy_hive.py#L176-L178) does not support the precision and scale attribute which is supported [since hive 0.13](https://cwiki.apache.org/confluence/display/hive/languagemanual+types#LanguageManualTypes-NumericTypes).

possible implementation solution:

``` python
class HiveTypeCompiler(compiler.GenericTypeCompiler):
# [..]

def visit_NUMERIC(self, type_):
precision = getattr(type_, "precision", None)
if precision is None:
return "DECIMAL"
else:
scale = getattr(type_, "scale", None)
if scale is None:
return "DECIMAL(%(precision)s)" % {"precision": precision}
else:
return "DECIMAL(%(precision)s, %(scale)s)" % {"precision": precision, "scale": scale}

```