Clean up get() calls, remove commented out code

@Ignat99 PTAL.

As suggested by @dionyziz at https://github.com/OpenBazaar/OpenBazaar/pull/658#issuecomment-56653243 I created a PR out of my own code-review comments.
If `shipping_price` is `None`, this will return the correct result, but only due to the way comparisons work for `None`.

Here's another take, a bit more straightforward:
```
try:
    contract_body = json.loads(u"%s" % contract['contract_body'])
except ValueError as e:
    self.log.error("Problem loading the contract body JSON: %s", e)  # correct erroneous %e specifier
else:
    try:
        item_price = max(0, contract_body['Contract']['item_price'])
    except KeyError:
        item_price = 0

    try:
        shipping_price = max(0, contract_body['Contract']['shipping_price'])
    except KeyError:
        shipping_price = 0
```
The rationale for catching the exception is this: We are inside a `for` loop, processing many `contracts`. A possible error in one of them shouldn't break the loop prematurely, but should be logged. The exception to catch is `ValueError`; see https://hg.python.org/cpython/file/ee879c0ffa11/Lib/json/decoder.py