[FIX] implemented seek() for bgzf stream, fixed BAM index, fixed extra tests/demos

@holtgrewe: I found only 1 bam index test which succeeded even before I implemented seek(). I added a seek test to test sequential and random seeks. The bgzf stream (and the BAM file via setPosition()) supports ios_base::beg seeks (BAM virtual offset) and ios_base::cur seeks (number of bytes to skip forward)