Fix: Let JSON parser turn float-like values into Decimal

[Tobiaqs]

Fix for #108

[skyl]

So, if in not mistaken, every program that uses this library would have to change everything to decimal. Other exchange client libraries use floats. Do some avoid floats by using strings exclusively?

I think this change would break a lot of people.

unsupported operand type(s) for +: 'Decimal' and 'float'

[ericsomdahl]

I think we can support Decimal usage but as @skyl says we must be smart about backwards compatibility. A simple idea would be to add this handler as a argument to the constructor

def using_requests(request_url, apisign, json_parse_args):
    return requests.get(
        request_url,
        headers={"apisign": apisign}
    ).json(**json_parse_args)

class Bittrex(object):
    def __init__(self, api_key, api_secret, calls_per_second=1, dispatch=using_requests, api_version=API_V1_1, json_parse=None):
        self.json_parse = json_parse if json_parse is not None else dict()
        self.api_key = str(api_key) if api_key is not None else ''

defaulting object construction to using the float (empty dict) handler will preserve float usage where expected

[JosephMRally]

Hello, thanks for implementing the fix. I feel that there needs to be a loud message about the dangers of using floats in high precision calculations.

Long ago i ran into the richard preyer superman half cent issue in an actual application, the effects become profound after several iterations of multiplications. Even today i fall now and then into this issue.

Here is an article on how using imprecise numerical representation caused 28 deaths and 100 injuries: http://www-users.math.umn.edu/~arnold/disasters/patriot.html

[maxmalysh]

Using floats for handling financial data is super dumb. It's a receipt for disaster.