Spark Payments (formerly steempay) - Amount Entry Screen [DEVELOPME...

Added the amount entry page. Before, the amount was static (set in the config), now a merchant can enter any amount in USD and the app will grab the latest exchange rate and charge the customer the correct amount of STEEM.

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Firstly, I had to add the markup, which is the number pad that the merchant uses to enter the amount:
https://github.com/SteemPay/spark-payments/blob/master/www/index.html#L50-L79
Next I had to add the CSS to style the number pad:
https://github.com/SteemPay/spark-payments/blob/master/www/css/main.css#L159-L209
And then I added these 2 methods to facilitate the adding and clearing of numbers:
https://github.com/SteemPay/spark-payments/blob/master/www/js/app.js#L27-L34

Among a few other minor changes to get things to work properly all contained in this commit:
https://github.com/SteemPay/spark-payments/commit/af931229cf6b6560cec30b4748a6608c9b009dea

Spark Payments (formerly steempay) - Amount Entry Screen [DEVELOPMENT]