Added: possibility to pass coin symbol instead of name (id) + URL fetching unified

[im-n1]

Hey there dude,

I've added coin symbol handling. Maybe someone would like to fetch "XRP" instead of "Ripple". Now he can. Check out the diff and let me know.

You can also use the method get_coin_name() separatelly like:

>>> from cryptory import Cryptory
>>> c = Cryptory(from_date="2013-01-01")
>>> c.get_coin_name("xrp")
'ripple'

I've also tried to unify URL fetching. Check out fetch_url() method.

P.S.
I would like to contribute much more because I'm now about to use your awesome library heavily. Maybe we can also make pep-8 happy, because now whole code shines like christmas tree under flake8 :D