Add an Android-only storage option for external storage

[paandahl]

Need new functions:

FileSystem.hasExternalStorage()
FileSystem.isExternalStorageAvailable()

And new storage-options:

FileSystem.storage.external

FileSystem.readFile() also needs to throw a recognizable exception if the storage has been removed.

[CptTony]

I second this request. I'm picking another library for this reason.

Will check back in on next project.