Update README (defadvice instead of add-function)

Fixes #gh-1 ("(package-refresh-contents) is not a valid place
expression").
Merge pull request #2 from sometimesfood/update-readme-defadvice

Update README (defadvice instead of add-function)
Use defadvice in comments
Merge pull request #3 from sometimesfood/update-comments-defadvice

Use defadvice in comments
Add support for different MELPA repo URLs

This commit adds support for multiple MELPA repo URLs and a dependency
on cl-lib (cl-find-if).
Merge pull request #4 from sometimesfood/support-multiple-urls

Add support for different MELPA repo URLs