tf.contrib.data is now tf.data

apis? --> APIs, eli jos on akronyymi isot kirjaimet, muuten toki pienellä käy
for others --> for other implementations?

The filename based annotation file can be turned on by passing
`--annotation_reader txt_file_reader_filename`.

The file did not work if the filenames were reused in different
directories. This change restricts the script to work only with datasets
that have the annotation and audio files in the same folder and have
the same filename other that the file extension.

This commit changes mfcc function to actually return the signal
spectrogram. This is done for testing. If the tests go well, I make the
preprocessing function a bit more configurable.

The transform-tfrecords.py sets the audio format to `.wav` when the
format should be `wav`. I fixed the bug in decoder because I didn't want
to recreate the dataset.