Minor change to gemspec summary field
Changing Reader attributes related to sample frames

Instead of sample_frames_read and sample_frames_remaining,
changing to current_sample_frame and total_sample_frames.

This seems more future proof if the ability to seek to arbitrary
sample frames (instead of reading all samples consecutively) is
added in the future. (As in, sample_frames_read and
sample_frames_remaining don't necessarily make total sense if
that is possible).

Also, querying the total sample frames seems like a more common
operation than querying the sample frames remaining, so this
has the benefit of making getting that more direct.
Some 'ternary if' magic for compactness
Consolidating fixture files to remove duplication

test/fixtures/expected_output (used by Writer tests) and
test/fixtures/valid (used by Reader tests) contained an almost
identical set of Wave file fixtures. Removed the /expected_output
folder in favor of using /valid.
Changing sample_count variable name to more accurate sample_frame_count
Minor README changes to reflect Reader sample frame attribute changes