M-* (pop-tag-mark) works after elpy-goto-definition

`M-.' in other language major modes is often bound to `find-tag', which,
after jumping to the tag's definition, allows you to use `pop-tag-mark'
(M-*) to jump back to the location where you invoked `find-tag'.

This commit makes `elpy-goto-definition' populate the same location
"marker ring" used by `find-tag' and `pop-tag-mark'.

Using etags.el's `find-tag-marker-ring' variable might be a bit hacky,
but the alternative is to create our own version of `pop-tag-mark',
duplicating ~20 lines of code from etags.el. I don't know how you tell
if a variable like `find-tag-marker-ring' is considered part of
etags.el's stable API (it isn't autoloaded, so maybe it isn't) but it
hasn't changed since 1998:
http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=0553b7d6
(note that etags.el is part of Emacs).
Merge pull request #89 from drothlis/pop-tag-mark

elpy-goto-location: Populate the M-* / pop-tag-mark location ring.
elpy-set-backend: Mention elpy-rpc-backend to avoid confusion.

Using elpy-set-backend in the .emacs does not work as expected, as
there is no Elpy backend at that point. The docstring now guides the
user to the correct configuration setting.

See #91.