#294 add button to generate a random playlist with 50 songs
remove print filecount
#294 use database for random calculation
#294 prevent usage of non unique random ids
#294 load random ids in one step
#294 correct usage of range and xrange ...
fix fore python3 (filter list)
Safe randomization for dirs and various corner cases
additional tests; readability

also: minor bug fix in debug output of database/sql
test coverage: min 54%, html output option

Minimum Coverage requirement for general tests raised to 54%.

`runtests` now takes an optional argument for the generation of an HTML
coverage report:

    $ runtests [-d OUTPUT_DIR] [TEST_TARGET]

The directory will be created if necessary.