Allow `sep` on `Angle.to_string` to be empty

Motivated by a discussion on astropy-dev with the subject ["How to create IAU standard object designation strings from astropy coordinates?"](https://groups.google.com/forum/#!topic/astropy-dev/J5b-4Oi-zj8) , this PR allows the `sep` option of `Angle.to_string to be an empty string.  This makes it much easier to do IAU-style names like ``SDSS J123456.78+123456.78``.

This also adds a couple of examples to the docs about how to use python's `format` to get pretty-looking coordinate strings.