Add proxy support in API definition

[daniele2010]

I've added the support of proxy, if you want to use a proxy you can specify it in the API definition as shown below:

import krakenex
k = krakenex.API(proxy='my.proxy.net')

[veox]

Ref issue: #6

[veox]

I'm reluctant to merge this, since v1.0.0a0 is already tagged for testing.

Also, once master is cleaned up for a proper release, and v1.0.0 actual is out, it's likely I'll just go on with the requests approach, as discussed in issue #11.

That should allow folk to use whatever way of connecting to Kraken that they see fit.

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Sorry for this! I'd usually close the PR at this point, but quite a few people seem to ask for this functionality. So, let's keep it open for now, and merge into v1.1.0.

[veox]

I've decided to instead push forward with the requests impl-n due to #53. It's now in master, and tagged as v2.0.0a1.

Sorry for dragging this out for so long.