Add discover-my-major

A better interface for getting information about major mode features. It can
complement the built-in "C-h m" (describe-mode): "C-h m" can be thought of
a full mode help, while discover-my-major a quick mode help, in the following
ways:

- Show key binding with description
- Cursor is automatically switched to discover-my-major help buffer, and to quit
the help buffer, simply press 'q'. In stock "C-h m", user has to manually switch,
scroll up/down to look for major mode commands. Finally, user closes the buffer
either by "C-x 0", or winner-mode undo (C-c <left>), which requires more key strokes.
- Display just enough information. In "C-h m", it displays every thing, from
major mode to minor modes, enabling minor modes and description of every modes.
- More polished user interface.

It's also better than smex-major-mode-commands, since smex-major-mode-commands
does not show key bindings and description.
Update documentation
Merge pull request #546 from tuhdo/master

Add discover-my-major