Add Azure storage option

[hongooi73]

Adds a cache_azure function, to put the cache in an Azure storage account (#93). Also includes tests, in testthat/test-azure.R.

Interface is simply

endpoint_object <- AzureStor::storage_endpoint("https://{acctname}.{storagetype}.core.windows.net", {auth_args})
azcache <- cache_azure("containername", endpoint_object)

Or alternatively,

azcache <- cache_azure("containername",
    "https://{acctname}.{storagetype}.core.windows.net", {auth_args})

where the endpoint object is created within the cache_azure call.

[hongooi73]

Also, the README says that the API should implement methods called exists and remove, but the actual methods are called has_key and drop_key (eg in cache_filesystem.R). I assume the README is outdated?

Nevermind, I see what's going on. Updated cache_azure to use the new-style caching mechanism.