Added support for selecting an account by alias instead of login.

Some accounts may have a confusing login and an alias is given for
clarification. The changes will use the alias in the completion
function instead of the login.
Updated emms-cover so full size album covers are stored in the album's
directory. Resized versions are still stored in a separate directory.
Removed shortcut for twitterer-mode.
Added autoload for the netrc-get function.