Use Composer to manage the dependency on phpseclib.
Write the server title and neighbor list.
Merge branch 'composer'
TPCStorage: Fix two possible PHP warnings.
TPCStorage: Require to specify the back-end class in the server array.

Much better than intransparently hardcoding the class choice depending on certain keys in the array.

This also allows "dummy servers" which have to handle their mirroring independently from the extension - a welcome change in view of the rather terrible SFTP back-end.