Support for BTC tesnet address generation

[reiven]

Hi, i will be nice to have an additional button to change the generator to "testnet" addresses.
I will try to change:

    var PUBLIC_KEY_VERSION = 0;
    var PRIVATE_KEY_VERSION = 0x80;

To testnet values (0x6F and 0xEF)

[brainwallet]

It's in the list of supported currencies, just select Testnet (TEST) from the list (upper right corner of the site).