add optional device parameter to read tests

This should make it possible to test manually, even if the default
device detection fails.
cmake: add valgrind target

Don't include this in the check target, since valgrind is quite a build
requirement *and* only works really well on Linux.
tests: allocate enough space for track_offsets

Not only the track offsets, but also the overal disc sector count is
saved in this array.
valgrind: also check for leaks

The default does display a summary, but we suppress this summary and it
is normally not indicated with an error status.
So we force returning an error code for leaks.
fix a leak in the tests

This is the memory problem I actually found first,
but valgrind pushed me to fix another one first ;-)
cmake: rename valgrind target to memcheck
detect SKIP in valgrind tests

Otherwise a SKIP is a failure.
memcheck: ignore non-skip exit code from discid

This way we can test discid/discisrc even when the tools themselve fail,
without having a special code for SKIP.
Stop using mm.musicbrainz.org, use a non-redirect URL for submissions
Fix tests
Merge remote-tracking branch 'lalinsky/no-mm'

see pull-request #33
update ChangeLog

Maybe we should release soon. These changes should be enough for a
release.
fix typo in test comparision

>= 0 is always true for unsigned int. We want > 0.
Found with coverity.
update ChangeLog for 0.6.0
LIB-48: don't return NULL on !success

This probably would lead to segfaults otherwise, which is quite
disruptive for something that can happen quite easily on usage errors.
Having empty values is probably a better indication on what is wrong.
fix ChangeLog to also include 0.5.2
update 0.5.2 ChangeLog
deactivate default_device test for now

This still doesn't work on Mac
no NULL for webservice_url, allow free(NULL)

While technically no object functions are supposed to be called on a
NULL object, calling discid_free(NULL) doesn't actually hurt and
forgetting to call free() is worse.

Changing the return on invalid usage (no previous read/put success)
was forgotten for webservice_url when implementing LIB-48.
correct 0.5.2 ChangeLog