Add Phablet.cmdline() and Phablet.connect()

Those two new methods make it possible to expose enough of phablet
internals to let applications invoke subprocess.Popen() directly and
interact with process I/O.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
add RemoteTemporaryDirectory

This new class works similarly to TemporaryDirectory but can be operated
remotely instead.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>