add FEATURES created from string array
doc: rename :novalue: to :annotation:

This might be in sphinx 1.2 as :annotation:
update features API as of libdiscid master
features API in python for libdiscid 0.3.x; re #20

To make sure we have a valid feature list,
we do some checking in Python when the libdiscid API
is not available, but ISRC/MCN API is.
DiscId.read() can raise NotImplementedError
FEATURES doc: + NotImplementedError
Merge branch 'features_string'

Add a list of FEATURES implemented on the current platform.
We build this list even if not given by libdiscid.

See #20
doc: we have >1 constants now
Merge branch 'annotation'

This is still not in upstream (sphinx), but the request was changed
to using :annotation:, rather than :novalue:.