Add coin_code to config variables that are returned from API

Makes it easier for pool UI to change UI/text rather than hard-coding "XMR" etc into UI views. For example:

UI template like:

`Balance: {{ balance }} XMR`

Can become:

``Balance: {{ balance }} {{config.coin_code}}``

lib/api.js

@@ -174,7 +174,8 @@ app.get('/config', function (req, res) {
         dev_donation: global.config.payout.devDonation,
         pool_dev_donation: global.config.payout.poolDevDonation,
         maturity_depth: global.config.payout.blocksRequired,
-        min_denom: global.config.payout.denom * global.config.general.sigDivisor
+        min_denom: global.config.payout.denom * global.config.general.sigDivisor,
+        coin_code: global.config.general.coinCode
     });
 });