Introduce epl-find-installed-packages

Make epl-find-installed-package obsolete
Add functions to work with built-in packages
Add epl-package-obsolete-p
Add epl-obsolete-packages
Merge pull request #26 from Silex/master

Add epl-upgradable-packages
Rename obsolete -> outdated
Include built-in packages in outdated tests
Call the right function

Thanks @Silex, for catching this!
Inline some trivial internal functions
Don't consider built-in packages in epl-oudated-packages

package.el doesn't consider built-ins as outdated either.
Add optional argument to check min-version of a package.