Replace redcarpet with kramdown for yardoc generation.

This allows building with jruby since kramdown does not use a C extension.
Support older libdiscid version (libdiscid versions 0.1.0 - 0.5.0 are supported).
Updated changelog

Signed-off-by: Philipp Wolfer <ph.wolfer@gmail.com>
More examples and better documentation.
Fixed reading from default device.

Signed-off-by: Philipp Wolfer <ph.wolfer@gmail.com>
Added DiscId.feature_list