stbt._X11Remote.press: Log keypress

Just like all the other remote-control implementations. This makes
debugging test failures much easier.

Arguably the logging should be done by `stbt.press`, but for example the
`_SamsungTCPRemote.press` implementation logs "Pressed" first and then
logs the reply; and `NullRemote.press` logs "Ignoring request to press".
Merge pull request #203 from drothlis/log-press

stbt._X11Remote.press: Log keypress