require auto-complete, fix issue #28

- requires `auto-complete` package. This is simpler to maintain and
  debug.
- remove `ac-source-yasnippet` from `stan-mode`
remove compilation-mode and flymake-mode support

New error message format a pain to parse. Remove support until I fix.
See issue #29
update-version.sh: fix checking for null string argument
update versions

- stan-snippets.el

  - added constants `stan-snippets-version`, `stan-snippets-lang-version`.
moved stan-snippets-mode support into stan-mode
update README.md

- only include documentation on how to install via package.el; remove instructions for manual install.
- update URL of MELPA
- add reference to Cask
- remove obsolete mentions to stan-snippets
added flycheck mode support, reintroduced compile-mode
READE: added flycheck-mode and compile-mode
example highlight: add reject
flycheck-stan: use a variable for the location of stanc
comments
use Cask for development
update Makefile to use Cask
add travis-ci support
add melpa badges
clean up compilation errors
remove with-no-warnings block; it wasn't doing anything
restore stan-snippets until melpa is updated
deprecate stan-snippets
misc