Updated all application urls to allow path prefix.

Allows BookStack to be installed at a non-root location on a domain.
Closes #40.

.env.example

@@ -3,6 +3,10 @@ APP_ENV=production
 APP_DEBUG=false
 APP_KEY=SomeRandomString
 
+# The below url has to be set if using social auth options
+# or if you are not using BookStack at the root path of your domain.
+# APP_URL=http://bookstack.dev
+
 # Database details
 DB_HOST=localhost
 DB_DATABASE=database_database
@@ -42,8 +46,6 @@ GITHUB_APP_ID=false
 GITHUB_APP_SECRET=false
 GOOGLE_APP_ID=false
 GOOGLE_APP_SECRET=false
-# URL used for social login redirects, NO TRAILING SLASH
-APP_URL=http://bookstack.dev
 
 # External services such as Gravatar
 DISABLE_EXTERNAL_SERVICES=false