Add IS_LINUX constant to Service class base

This will be used, at least initially, for Festival, as we are only
writing in Linux support at first.
Split off path_input() from path_workaround()

While the latter is used for Windows to determine if an input file is
needed as a workaround for unicode errors, the former is now available
when an input file is ALWAYS needed for a service.
Remove TODO for possible urllib2 exceptions

These are correctly handled in the router.
Basic Festival support

Implements support for voices, only, in this first pass.
Festival support for volume

Also removes the possible TODO for frequency, which does not seem to be
something worth doing from here.
Merge branch 'add/festival-service' into develop

Adds support for the Festival service, on Linux only for the time being.