Make the name field in projects optional (#870)

Closes #858.

If a project is named `default.project.json`, it acts as an `init` file
and gains the name of the folder it's inside of. If it is named
something other than `default.project.json`, it gains the name of the
file with `.project.json` trimmed off. So e.g. `foo.project.json`
becomes `foo`.

rojo-test/serve-tests/no_name_default_project/src/third-level/default.project.json

@@ -0,0 +1,8 @@
+{
+  "tree": {
+    "$className": "IntValue",
+    "$properties": {
+      "Value": 1337
+    }
+  }
+}