Support folder hierarchies in store.

[itsfarseen]

./age-store.py add-file foo/bar should create the folder store/foo before writing store/foo/bar.enc

See: #2 (comment)

[itsfarseen]

See #7 for ongoing discussion about this feature.

[itsfarseen]

We should think about how this feature would interact with the current interface of age-store, and how it may affect future extensibility. We should design the API to not conflict with any future extensions we may want to introduce.