Closes #497: Allow functions as elements in yas-key-syntaxes

* yasnippet.el (yas--templates-for-key-at-point): Renamed from
`yas--current-key'.
(yas-key-syntaxes): Overhaul documentation.
(yas-expand-from-trigger-key): Use
`yas--templates-for-key-at-point'.

* yasnippet-tests.el (complicated-yas-key-syntaxes): New test.
(yas-should-expand, yas-should-not-expand): Friendlier failure
message.
* yasnippet.el (yas-key-syntaxes): Improve documentation.