Fix several bugs and add two features.

Assertion error message had typos in arguments to string format.

.cover files for modules in packages are now put in the right place.

The code that generate .cover files seemed to prepend a "./" to many
absolute paths, causing them to fail.  The code now checks explicitly
for absolute paths and leaves them alone.

In trace/coverage code, recover from case where module has no __name__
attribute, when e.g. it is executed by PyRun_String().  In this case,
assign modulename to None and hope for the best.  There isn't anywhere
to write out coverage data for this code anyway.

Also, replace several sys.stderr.writes with print >> sys.stderr.

New features:

-C/--coverdir dir: Generate .cover files in specified directory
instead of in the directory where the .py file is.

-s: Print a short summary of files coverred (# lines, % coverage,
name)