fix minor mode keymap prefix and allow customization (fixes #137)

The official emacs key binding conventions state:

> Don't define C-c letter as a key in Lisp programs. Sequences
> consisting of C-c and a letter (either upper or lower case) are
> reserved for users; they are the only sequences reserved for users,
> so do not block them.
>
> Changing all the Emacs major modes to respect this convention was a
> lot of work; abandoning this convention would make that work go to
> waste, and inconvenience users. Please comply with it.

but org2blog defines the following keys:

    (define-key org2blog/wp-map (kbd "C-c p") 'org2blog/wp-post-buffer-and-publish)
    (define-key org2blog/wp-map (kbd "C-c P") 'org2blog/wp-post-buffer-as-page-and-publish)
    (define-key org2blog/wp-map (kbd "C-c d") 'org2blog/wp-post-buffer)
    (define-key org2blog/wp-map (kbd "C-c D") 'org2blog/wp-post-buffer-as-page)
    (define-key org2blog/wp-map (kbd "C-c t") 'org2blog/wp-complete-category)

This commit makes the prefix customizable via a new `org2blog/wp-keymap-prefix` variable, whose default is `"C-c M-p"`.  After changing it, `org2blog/wp-reload-entry-mode-map` must be called before it takes effect.