Added auto_round and always_float attributes to DecimalField
Not sure on the application of the float option, since there's a floatfield already.  As far as rounding, I suppose that would be handled by the DB when you specify a precision on the column, but this might need to be looked into a little more.
 - Yes there is `FloatField` but it does not allow us to define number of decimals I need, (e.g. only two for currencies). Because of dynamic nature of Python it is difficult to work with codes where there is a possibility of mixing `float` data with the `Decimal` ones. So, it is important to normalize them to one type - maybe float. Well now I agree that in such a scenario it would indicate a design issue or there should be central point where the code should convert the `Decimal` returned to `float`, but then again I don't see anyway to implement custom getters and setters in Peewee `Model` to do it centrally. I will leave this open to your good judgement.
- DBs are responsible to accurately store the data, so if any data passed to it needs to be rounded which will result in loss of precision then it is going to throw a warning. This can happen when our `Decimal` has more decimals, or if we pass a `float`. Storing `23.221` in a `Decimal(5,2)` column in MySql throws 'Error 1265: Data truncation' warning, and I don't see any way to turn this off in DB. Even this warning is treated as error in Peewee, disrupting normal flow. By specifying `decimal_places` user has already consented with fact that he wants only that many decimals. Peewee already has enough information to lend user this helping hand. I really do not like warnings in my code. Now it can be argued that some users might want to warned about this loss. That is why I have added the `auto_round` flag to let user turn this behavior on or off. In fact it is off by default so that Peewee's current behavior do not change.
 Added in 4c3af4ccc83dad38a369a1b8e3121de1d6838a7b -- thanks for the help!

- auto_round makes sure that if the value has higher precision than the
  number of decimal_places then it round it to that value before send
  that to db. This will save from some unexpected "Data truncation"
  warnings from MySQL (http://bit.ly/bWr1mn).
- always_float makes sure that in Python code always gets the value as
  float instead of Decimal. This way the return value can be easily
  intermixed with other floats. Also float can be faster than Decimal.
