Closes #495: Allow symbols as aliases in `yas-snippet-dirs'

* yasnippet.el (yas-installed-snippets-dir): New parameter. Set at
load time from `load-file-name'.
(yas--load-file-name): Removed.
(yas-snippet-dirs): Update docstring. Default value has symbol
`yas-installed-snippets-dir' as an alias to any bundled snippets.
(yas-snippet-dirs): Redesign for new `yas-snippet-dirs' semantics.