Modular data transfer options

[sbliven]

Refactor the library to allow multiple data transfer options. Eventually this will include none (--central), rsync, globus, and S3.

For this ticket, refactor the existing rsync code to make an abstract interface for the API. New methods should be discussed under their own issues.

There should be a consistent way of choosing and configuring the data transfer code from the command line, especially since some methods will need more complex authentication and configuration than the current rsync method.