Add I2P client plugin

[str4d]

The I2P equivalent of ticket #246. The I2P client plugin should be highly configurable yet work with a best effort if the user supplies zero parameters. It could potentially launch I2P if an I2P installation is present in the path, or use an already-running I2P if a SAM port is specified (or the default is active).

[Brian Warner]

Cool. Take a look at ​https://github.com/warner/foolscap/pull/27 for how I'm looking to structure the Tor connection handler. Please consider making a couple of separate functions to split out the different use cases (use typical default ports, connect to explicit pre-running daemon, launch new daemon), if that seems appropriate for I2P, rather than having a single function with a whole bunch of mutually-contradictory arguments.

[str4d]

Implemented in ​https://github.com/warner/foolscap/pull/28

Currently only supports already-running I2P, pending support in txi2p for launching I2P and parsing local config.

[Brian Warner]

Moving these tickets from 0.12 to 0.13, since we need 0.12 out quickly to resolve #263

[Brian Warner]

I've landed str4d's PR (with changes) in [7f0fa3e6a096855e7fac51e481ddc5d8580bdd4c]. He had run manual tests with an earlier version, but I changed a couple of things since then, so I'd like to get some new test results. I'm going to leave this ticket open until we've had a chance to make sure it works properly.

[Brian Warner]

str4d told me on IRC that the PR has been tested manually, and works (with a slightly modified version of test/check-connections-client.py / server.py). So I think we can close this one now.