Add AMCREST_POLL_STORAGE option

[benhoman]

Added an option amcrest_poll_storage to allow a user to skip polling storage information. This is helpful for when a device doesn't have any storage.

This would avoid #17

[dchesterton]

Hi Ben, I've implemented it in a slightly different way by adding a STORAGE_POLL_INTERVAL environment variable which can be set to 0 to disable the polling, but thank you for your PR, it's much appreciated.