bt2: add support for comp. class help

bt2: add support for comp. class help

The description of a user component class is the first line of the
class's docstring; the help is everything else starting at line 3:

class MySink(bt2.UserSinkComponent):
    '''This is the description.

    This is the help.

    This too:

        * And this.
        * And also that.

    Voilà.
    '''

    def _consume(self):
        # ...

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>