add discid_get_support API (string array)

This is part of the implementation for LIB-23
http://tickets.musicbrainz.org/browse/LIB-23
add linux features
add feature list for other platforms
use feature tests in the discisrc example

For use in C the discid_has_feature() function is recommended,
together with the values in the discid_feature enum.
feature names to lowercase; room for 32 features

It is a lot easier to get the feature names right
when you don't have to think about casing.

We will probably never reach 16 features,
but better safe than sorry.
Having an array of 32 pointers doesn't hurt.
Additionally, that is the same room we would have
when using bit masks in an int (mostly 4 byte).
set feature values to powers of 2 (bit mask)

This enables us to set multiple features in one int
for possible implementations for LIB-29.
(read_sparse)
Merge branch 'features_string'

This adds an enum discid_feature (as bit mask)
discid_has_feature() and discid_get_feature_list()
to the external API.
has_feature is intended for C users
and the feature_list could be of help for some bindings.