fixed PrivateKey initialization from JSON

[mpolci]

I discovered this error after calling toWIF() method on privateKey initialized from a json text like this:
{"bn":"ce6...fec","compressed":true,"network":"testnet"}
The network was not initialized with a Network object but toWIF() needs to read network.privatekey.

[coveralls]

Coverage increased (+0.0%) to 97.97% when pulling d60d8cf on mpolci:master into c671a97 on bitpay:master.

[braydonf]

I think we should add a test that checks instantiation from JSON if it doesn't already exist.

[mpolci]

I wrote the test

[coveralls]

Coverage increased (+0.0%) to 97.97% when pulling 0d2784e on mpolci:master into c671a97 on bitpay:master.

[pnagurny]

LGTM