Closes #470, handle non-whitespace newlines

Many modes put newlines in comment ender class because they have line
comments.

* yasnippet.el (yas-try-key-from-whitespace): new function.
(yas-key-syntaxes): use it in place of "^ ".
* yasnippet-tests.el (complicated-yas-key-syntaxes): test it.
Add yas-shortest-key-until-whitespace

Example yas-key-syntaxes function.

* yasnippet.el (yas-shortest-key-until-whitespace): new function.
* yasnippet-tests.el (complicated-yas-key-syntaxes): test it.
Pass start point to yas-key-syntaxes functions

* yasnippet-tests.el (complicated-yas-key-syntaxes): accept argument.
* yasnippet.el (yas-try-key-from-whitespace): accept argument.
  (yas-shortest-key-until-whitespace): accept argument.
  (yas--templates-for-key-at-point): pass start point the methods.
  (yas-key-syntaxes): update docstring.
Add yas-longest-key-from-whitespace

Example yas-key-syntaxes function.

* yasnippet.el (yas-longest-key-from-whitespace): new function.
* yasnippet-tests.el (complicated-yas-key-syntaxes): test it.
Fix yas-key-syntaxes example functions docstrings.

* yasnippet.el (yas-try-key-from-whitespace),
  (yas-shortest-key-until-whitespace),
  (yas-longest-key-from-whitespace): docstring describes the effect, not
  implementation.
Fix edge case in yas-longest-key-from-whitespace

* yasnippet.el (yas-longest-key-from-whitespace): don't try again if
  start-point is on whitespace.
* yasnippet-tests.el (complicated-yas-key-syntaxes): check edge case.
Merge pull request #503 from npostavs/key-syntaxes

More yas-key-syntaxes functions