Feature/sp call limits (#63)

* add api for setting/querying call limits by account and sp

* update an account or sp limit if one exists rather than creating a new one

lib/models/account-limits.js

@@ -1,9 +1,17 @@
 const Model = require('./model');
+const {promisePool} = require('../db');
+const sql = 'SELECT * FROM account_limits WHERE account_sid = ?';
 
 class AccountLimits extends Model {
   constructor() {
     super();
   }
+
+  static async retrieve(account_sid) {
+    const [rows] = await promisePool.query(sql, [account_sid]);
+    return rows;
+  }
+
 }
 
 AccountLimits.table = 'account_limits';