Convert decimals from SQL results

[balloob]

Description:

Some adapters for SQL Alchemy will return data wrapped in Decimal. This should not be added to the state machine as it blows up the JSON conversion.

Related issue (if applicable): fixes #13021, #13050

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[juan11perez]

Good day. Updated to 65.2, frontend loading. sql sensor still generating similar log:

2018-03-10 20:46:42 ERROR (MainThread) [homeassistant.components.sensor] sql: Error on device update!
Traceback (most recent call last):
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/helpers/entity_platform.py", line 188, in _async_add_entity
    await entity.async_device_update(warning=False)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/helpers/entity.py", line 327, in async_device_update
    yield from self.hass.async_add_job(self.update)
  File "/usr/local/lib/python3.6/asyncio/futures.py", line 327, in __iter__
    yield self  # This tells Task to wait for completion.
  File "/usr/local/lib/python3.6/asyncio/tasks.py", line 250, in _wakeup
    future.result()
  File "/usr/local/lib/python3.6/asyncio/futures.py", line 243, in result
    raise self._exception
  File "/usr/local/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/sensor/sql.py", line 147, in update
    value = float(decimal)