Improve how CSS and JS is added to the theme.

Split CSS and JS enqueue into two different functions.
Make both functions check for existence to enable a child theme to override it if needed (http://codex.wordpress.org/Child_Themes#Using_functions.php).

Signed-off-by: Paul Vincent Beigang <pbeigang@gmail.com>
Merge pull request #15 from pablopaul/improve-js-css-enqueue-functions

Improve how CSS and JS is added to the theme.