Allow color-theme or deftheme switching.

Set theme-changer-mode to "deftheme" to use the Emacs 24 theme
facility. The default remains "color-theme".
Update README.md to include theme mode selection.
Merge pull request #2 from mfisher911/master

Allow Emacs 24 "deftheme" mechanism.
Use "deftheme" by default
package.el conformance