Don't abort installation when 'apt-get update' encounters an issue.

Hopefully it always works, but it's not a fatal issue. When it could be
a problem, we should deal with it the best we can.
Don't abort installation when firmware fails to install, only report it.
Hard code raspberrypi.org release to wheezy in sources.list.

While raspbian does support multiple releases (wheezy and jessie atm),
the raspberrypi archives do not.
Therefor hard code wheezy for the raspberrypi.org archives.
Place downloading of package files into its own function.

This will make it easier to make it more robust by trying various urls
if one compression format is not supported (anymore).
Added support for various compression formats for Packages download.

Now xz (apparently the new default), bzip2 and gz are supported.