allow read/put use at module level

Not exactly sure, why I decided using "del" wouldn't be good
enough, but having the option of an easier interface is no bad thing.

It is an open question if we should allow read/put on module and on
class level or hide the class methods.
document discid.read/put on module level

These should now be used to create DiscID objects,
rather than creating them directly.
fix free() for tests
change examples to module level read()

This also means the DiscId() object is not mentioned in code directly
anymore.
rewrite tests to use read() on module level
Merge branch 'easy_free'

See pull-request #22