Fixed integration tests.
Use google-translate-listen-program.

By default it is nil. User must put there the program to listen
translations. For example user can put there "mplayer". Of course
mplayer needs to be installed and needs to be located somewhere in the
PATH.
Add doc for google-translate-listen-button-face
Update doc for `google-translate-listen-program'
Minor change
Fix wrong doc string
Add documentation for `google-translate-listen-button-face'
Update documentation for `google-translate-listen-program'