`coverage` updates appear to have changed the reporting defaults. This
commit changes the reporting code to explicitly request missing
statements in the report.

`unittest` appears to no longer be raising SystemExit on completion.

SystemExit re-raised to exit with the correct return code.

Added an assertion to check test_generator has be instantiated
correctly, instead of the class itself being used as a decorator.

Generate a single, unnumbered, zero-argument test method by creating
a test_generator decorator with no arguments or the first argument as
None.

To make coverage tests work, build to a build path and add it to
sys.path, remove tested modules before tests so that the import-time
code is included in coverage.

If reporting coverage to stdout, don't close it once the report is
complete.

Added bytes & bytearray as iterable types that are not to be recognised
as positional arguments when interpreting test_generator profiles.

The filename attribute is not present on all loader classes, the
get_filename() method is the correct way of getting a module's file
name.

_is_args_kwds() was checking for two sets of positional arguments,
changed to the correct behaviour of one positional, one keywords.

In generator.test_generator._is_args_kwds, the first subcondition
( len(obj) == 2 ) can match dicts with two items, in which case (unless
the keys for the two items happen to be 0 and 1) the second two
subconditions will raise KeyErrors. This is now treated in the same way
as a TypeError.

Instead of storing the test_generator in-place of the function it wraps
and later swapping the function back during metaclass processing, the
generator is now stored as an attribute of the function.

One of the arguments of unittest_extra.generator.test_generator.wrap()
was confusingly named 'instance' even though it was a actually a class
object.

unittest_extra.generator.test_generator.wrap() has been changed to take
an instance rather than a dictionary of attributes, allowing it to use
inheritance features to get attributes from bases.

Due to Python 2's stupid implicit relative imports, changed the name
so that it no longer conflicts with the top-level package 'coverage'
which we need to import. (Circular imports - yay).

In coverage.coverage, don't attempt to import packages who's filenames
match patterns in 'exclude'.