add seconds attributes and length/sectors aliases

We now have sectors and seconds on Disc and Track level.
The length is an alias to sectors, even though
for tracks the attribute is called length in libdiscid.

Having Disc and Track working the same might be easier to understand
though.
add tests for sectors/seconds
lessen the API warning
Merge branch 'seconds'

See pull-request #28

only a small conflicts on imports

Conflicts:
	discid/disc.py