Add Binance International

[lthiery]

A new method of single-shot reporting by weighted average. It looks like this:

./helium-wallet oracle report-weighted-average --binance-us 1 --binance-int 2
Binance US                reports price of $1.1491
Binance International     reports price of $1.14958000
Report price 1.149419885058000 @ block height 553939?
Enter password to confirm.
Password: 

In addition, the user can set the helium-wallet up to report every N minutes, requiring only an unlock of the wallet at startup.

[lthiery]

This is what the automated interface looks like

./helium-wallet oracle automate --delay 20 --binance-int 1 --binance-us 1`
Starting oracle report automation with the following weights: 
Weights { binance_us: 1.0, binance_int: 1.0, bilaxy: 0.0, coingecko: 0.0 }
Enter password to start utility. 
Password: [hidden]
Binance US                reports price of $1.14
Binance International     reports price of $1.15
+--------------+---------------------------------------------+
| Key          | Value                                       |
+--------------+---------------------------------------------+
| Block Height | 553970                                      |
+--------------+---------------------------------------------+
| Price        | 1.145                                       |
+--------------+---------------------------------------------+
| Hash         |  HASH HERE                                  |
+--------------+---------------------------------------------+

After that, it will sleep for $delay minutes before starting back at the Binance US line.

[lthiery]

To make further arbitrage more challenging, I've also made it easy to create random delay between reports. This uncertainty should increase risk of any strategies that monitor and anticipate oracle behavior.

[lthiery]

One final feature of this PR is that the automated submission can specify a percentage difference that would trigger a quick submission. This will decrease on-chain latency in the event of an arbitrage opportunity.

[lthiery]

Any higher level features are being deferred until a longer-term oracle app strategy is figured out. This PR only adds Binance International as a price source.