Decimal type support is missing

Support for decimal type is missing from ParamEscaper.escape_item

Reproduce:
```
engine = create_engine(url)
q = text('select * from test where a = :param')
q = q.bindparams(bindparam('param', Decimal('1.0')))
# Raises exc.ProgrammingError "Unsupported object 1.0"
engine.execute(q)
```