remove bundled dropdown-list.el

It is available from melpa, so no need to bundle it.
Merge pull request #370 from tarsius/master

remove bundled dropdown-list.el
do not require dropdown-list at compile time

It might not be available.

Also update commentary with respect to dropdown-list.
Merge pull request #377 from tarsius/master

do not require dropdown-list at compile time
Closes #318: now checks for circular parenthood in snippet dirs
Minor: indenting in for yasnippet-tests.el macro
Closes #372: removed trailing spaces in snippets
doc: remove README.org
doc: finally decided to write the manual in markdown

will use same or similar doc helpers
closes #391: don't assume `declare-function'/ exists, it doesn't in emacs22
add support for visit-snippet-file.