Add option for verbosity level.

[ucarion]

facebookarchive/libwit#1 added a verbosity argument to libwit, but libwit-ruby unconditionally sets this value to 4, the highest possible level.

This PR adds an optional supplemental argument to Wit#init that lets users specify their desired log level:

# The `nil` is to use the default device_opt, the `0` tells Wit not to log at all
Wit.init(nil, 0)

I haven't put any documentation in because I'm not sure where that would go here.

[onionjake]

+1

[georgecoltart]

+1

[patapizza]

Just added -- see #4