plugin_registry: fix issue with non-discoverable plugins

There are "native" plugins, such as XML under `format` module.
The problem with these is that once they are dropped
(`PluginRegistry.setup(reset=True)`), they cannot be re-discovered
any more (the first discovery is usually implicit through
the import + metaclass machinery).

Now we can also remove a hack in `PluginRegistry.discovery()` that used
to inject such "native" plugins implicitly (but till the whole registry
got restarted!) if full search was requested.

Thanks to Travis CI for exposing the issue because of the different
ordering of the unit tests (subsequently reproduced at me with
`./run-tests tests.{format_manager.Injection,command_manager.Default}`).

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
plugin_registry: drop unused class attribute

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
run-tests: avoid specifying "tests." if enum'ing modules

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
README: fully switch to rst format + extend a bit

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
setup.py: cond_require() to deal with Python 2.6/2.7

+ extra cosmetic cleanup

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>