add support for 8 byte big endian nonce to Message

...network protocol is backwards incompatible with previous network protocl.
also includes a second version number for backwards-compatible changes (the
original version number was for backwards-incompatible changes).
add nonce support to WebRTC and Wallet

Each person keeps track of their own nonce, and the nonces of the other
copayers. The nonce is iterated for each message. If a person ever doesn't
iterate their nonce, that message is discarded by the copayers.

The nonces are saved as networkNonce (your nonce) and networkNonces (the nonces
of your copayers) in the wallet file.

In order to support restoring old wallets, the first four bytes of the 8 byte
nonce are actually the current time in seconds. Thus you can restore an old
wallet, because certainly at least one second has passed since your last
message. Only if you try to restore an old wallet within 1 second from the time
of your last message will you have a problem (or if your system clock is
grossly inaccurate).
change encryption to work on hex strings

..instead of binary, to work around an issue with bitcore/sjcl. I'm not sure
what the issue is exactly, except that encryption of binary data isn't working
correctly due to some kind of string stuff involving decodeURIComponent inside
sjcl. I haven't fully figured it out. For now I am changing the network
protocol to hex to workaround the issue.

See this: https://github.com/bitpay/bitcore/pull/416
Merge pull request #881 from ryanxcharles/feature/nonce

WIP: Add nonce to network protocol