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
Fix define-key 'help-command
Merge pull request #551 from toctan/fix-define-key

Fix define-key 'help-command
Extend improvements section
Remove EmacsWiki section
Leftover from previous commit
Log nREPL messages by default
Do not bind proced with OS X
Get rid of the deprecated clojure-test-mode
Add extra keybindings for eval-defun and eval-buffer
Don't recompile elc files outside of Prelude
Update document and operate-on-number key bindings

Signed-off-by: Tu Do <tuhdo1710@gmail.com>
Merge pull request #554 from tuhdo/master

Update document and operate-on-number key bindings
Move the prelude tips for key-chord-mode to it's module file.
Merge pull request #555 from hbin/prelude-tips-for-key-chord-mode

Move the prelude tips for key-chord-mode to its module file.
1. Rename prelude-indent-region-or-buffer to prelude-indent-buffer-or-region.
2. Add prelude-cleanup-buffer-or-region.
3. Add prelude-untabify-buffer-or-region.
4. Update menus and kbds.
Merge pull request #558 from hbin/operations-on-buffer-or-region

Operations for buffer or region
Merge pull request #553 from toctan/proced-binding

Binding proced only in Linux