Update Digit Truncation to Rounding

At the moment, if more digits than the normally allowed maximum are specified, additional digits will be truncated away rather than being rounded. This may seem small, but for those writing automated traders based on this library, it might be significant after many trades. Added in a round() before converting the value to a string and truncating. Please merge if you feel appropriate.
Update common.py
Merge pull request #5 from LolUsernameTaken/patch-1

Update Digit Truncation to Rounding--thanks for the patch!

Over the weekend I think I'm going to fix the underlying problem--I'm implicitly using floating-point values (because I didn't tell the JSON parser not to), which is bad for handling currency.  Ideally, the handling of amounts should be exact to either the number of decimal places allowed by the API, or by the currency (8 for BTC, for example) as appropriate.  Hopefully that won't break too many people.