Add eslint rules to enforce changes

Author: dfilipidisz

.eslintrc

@@ -88,7 +88,9 @@
         // Strict Mode
         "strict": [2, "global"],
         // Variables
+        "prefer-const": 2,
         "no-catch-shadow": 2,
+        "no-const-assign": 2,
         "no-delete-var": 2,
         "no-label-var": 2,
         "no-shadow": 2,
@@ -98,6 +100,7 @@
         "no-undefined": 2,
         "no-unused-vars": 2,
         "no-use-before-define": 2,
+        "no-var": 2,
         // Stylistic Issues
         "indent": [2, 2, {
           "SwitchCase": 1