drop support for usernames

Mojang has disabled their legacy skins API:
https://twitter.com/MojangSupport/status/964511258601865216

With their API rate limits, it's now practially impossible
for us to support usernames.

Fixes #142. The default parameter allows using:

- UUID
- URL
- MHF_Alex
- MHF_Steve
- Alex
- Steve

Contrary to UUIDs, using alex/steve doesn't redirect
and instead provides the skin from a locally stored file.

lib/routes/avatars.js

@@ -79,7 +79,7 @@ module.exports = function(req, callback) {
   } else if (!helpers.id_valid(userId)) {
     callback({
       status: -2,
-      body: "Invalid UserID",
+      body: "Invalid UUID",
     });
     return;
   }

lib/routes/capes.js

@@ -20,7 +20,7 @@ module.exports = function(req, callback) {
   if (!helpers.id_valid(userId)) {
     callback({
       status: -2,
-      body: "Invalid UserID"
+      body: "Invalid UUID"
     });
     return;
   }

lib/routes/renders.js

@@ -94,7 +94,7 @@ module.exports = function(req, callback) {
   } else if (!helpers.id_valid(userId)) {
     callback({
       status: -2,
-      body: "Invalid UserID"
+      body: "Invalid UUID"
     });
     return;
   }

lib/routes/skins.js

@@ -78,7 +78,7 @@ module.exports = function(req, callback) {
   if (!helpers.id_valid(userId)) {
     callback({
       status: -2,
-      body: "Invalid UserID"
+      body: "Invalid UUID"
     });
     return;
   }