Add a Mosh (mobile shell) component https://mosh.org/ , especially for the "local echo" feature ( see [demo](https://youtu.be/XsIxNYl0oyU?t=6m11s)) that makes remote shell over high latency network less painful.

---
I could not get the integrated `mosh` command to work with webrunner's openssh, but I'm already opening this MR for visibility.
For now, connecting using the way described as **Q: How do I run the mosh client and server separately?** in mosh [FAQ](https://mosh.org/#faq) works.

You can evaluate this with:
```ini
[buildout]
extends = component/mosh/buildout.cfg
parts = mosh
```

Start the server from the server side (after ssh'ing to the server) :
```bash
$ LC_ALL=C.UTF-8 ./parts/mosh/bin/mosh-server new -s -v
```
it will prints something like:
```
MOSH CONNECT 60002 XXXeu5nj+CLZtw/HiIcrLQ

mosh-server (mosh 1.3.0) [build mosh 1.3.0]
Copyright 2012 Keith Winstein <mosh-devel@mit.edu>
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[mosh-server detached, pid = 24525]
```

then from client connect with:
 ```bash
$ MOSH_KEY=XXXeu5nj+CLZtw/HiIcrLQ mosh-client 2001:X:X:X:X  60002
```

/reviewed-on !164