Create device model and setup devise, activeadmin and rspec!

- with attributes: model, imei_number, unique_id and access_token
 - access_token is automatically generated on create

app/assets/stylesheets/active_admin.scss

@@ -0,0 +1,17 @@
+// SASS variable overrides must be declared before loading up Active Admin's styles.
+//
+// To view the variables that Active Admin provides, take a look at
+// `app/assets/stylesheets/active_admin/mixins/_variables.css.scss` in the
+// Active Admin source.
+//
+// For example, to change the sidebar width:
+// $sidebar-width: 242px;
+
+// Active Admin's got SASS!
+@import "active_admin/mixins";
+@import "active_admin/base";
+
+// Overriding any non-variable SASS must be done after the fact.
+// For example, to change the default status-tag color:
+//
+//   .status_tag { background: #6090DB; }