Add exchange_rates()

This is the first function added that does not require auth credentials.
It is available both on the coinbase module and on CoinbaseAccount.
Make CoinbaseAccount auth optional, clean up stuff
Add @property to exchange_rates
Add allow_transfers flag to CoinbaseAccount (default false)
raise more specific exception
make CoinbaseError an Exception
delete empty README.txt
Change allow_transfers default to True
expand example usage a bit
spelling ("guarantee")
whitespace
rename global_request_params to auth_params
don't include the api key in unauthenticated requests
move tests.py -> tests/__init__.py
test with exchange_rates.json
move test oauth credentials to json file
move button test json to button.json
use Decimal for CoinbaseAmount
add trailing comma
add missing 'mock' test dependency