Add file caching app

Convert document image cache to use file cache manager app.
Add setting DOCUMENTS_CACHE_MAXIMUM_SIZE defaults to 500 MB.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/file_caching/admin.py

@@ -0,0 +1,10 @@
+from __future__ import unicode_literals
+
+from django.contrib import admin
+
+from .models import Cache
+
+
+@admin.register(Cache)
+class CacheAdmin(admin.ModelAdmin):
+    list_display = ('name', 'label', 'storage_instance_path', 'maximum_size')