Bump version to 0.7.1
Set attack phase even if no sequences are active

Otherwise the state phase is never set to attack which prevents releasing the
track note.
Remove unused code
Add comment
Check sustain phase length of sequence

A sustain phases with length 0 would cause an inifinite loop.
Reduce unnecessary sequence loops
Add BK_SAMLE_RANGE attribute

Remove BK_SAMPLE_OFFSET and BK_SAMPLE_END in favour of the new attribute.
Bump version to 0.8
Fix instrument sequence setter function

Sequence object would have been invalid if an error occured in the creation
function.
Fix off-by-one error

Too much shift caused integer overflow.
Fix behaviour of empty release envelope sequences
Fix next return value of `BKSequenceFuncSimpleStep`
Silence compiler warnings