Add preliminary support for managing sound files

[rytilahti]

• Adds install_sound which expects url, md5sum and id
• Adds printing of current sound volume to sound output.

[rytilahti]

Requires testing & updating the docs accordingly, an open question is what should be used as a "id" (the current one can be queried with 'sound') for newly installed ones. Another open question is how to switch between languages without doing a "full" installation.

[coveralls]

Coverage decreased (-0.2%) to 62.864% when pulling 10ce6e9 on sound_commands into dd76fcd on master.

[coveralls]

Coverage decreased (-0.3%) to 62.738% when pulling 3d5cac2 on sound_commands into dd76fcd on master.

[coveralls]

Coverage decreased (-0.3%) to 62.738% when pulling 124c791 on sound_commands into dd76fcd on master.

[rytilahti]

I think this can be merged now, I just tested out generating an audio pack and then installing it afterwards. To simplify the process, the server functionality worked in #153 will be used by this functionality when its done.

To add to the earlier comments by myself, looks like the id can be chosen as you wish (however, passing e.g. 0 gives an error). Furthermore, there seems to be no way to change the sound pack without a download; spamming sids will likely cause filling up the hard drive of the robot.

[coveralls]

Coverage decreased (-0.3%) to 62.738% when pulling 05b5e9a on sound_commands into dd76fcd on master.