Add todo list [ci skip]

Author: dahlia

TODO

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+- Python 3 (depends on Wand...)
+- Storage migration tool e.g. migrate(metadata, src, dst)
+- Routing storage e.g. RoutingStorage({object_type: store, ...})
+- Widely-used backends implementations
+  - Windows Azure Storage
+  - Rackspace Cloud Storage
+  - Google Cloud Storage
+  - imgur (eh?)
+- More docs
+- More tests