Parameterize the location of the server in the generated Python/JavaScript code

[exarkun]

Both the Python and JavaScript code generators assume that the location of the server will be known at generation time and that there will be only one. Neither of these assumptions holds true for Tahoe-LAFS storage servers.

Fix the code generator so that the location of the server is parameterized somehow. Allow clients to select which server they are talking to at runtime and to talk to more than one of them in the same program/process.