Stop Flycheck globally when Emacs is killed [#366]

A local kill-emacs-hook doesn't make sense, and didn't really seem to
work anyway.

Now we add a global hook, and iterate over the entire buffer list to
stop Flycheck in any buffer where it's active.
Specify package files in Cask
Ignore the dist package generated by Cask
Update the Makefile to use new Cask 0.6 features

Cask will build our package now, including the info files, so we don't
need our own stuff anymore.  Hence we drop our own info and package
targets.

We also refactor and improve the makefile to make the target name a bit
more consistent.
Update the manual
[Fix #367] Handle Clang errors from included files
Use Cask 0.6 on our testing VM
Fix some small checkdoc issues
Fix cleanup targets in makefile
Fix travis builds
Release Flycheck 0.18
Bump version in master
Fix wording
Build docs in /build/
Move installation instructions in their own doc
[Fix #336] Add Quick Start section
Prominently reference the changelog
Fix lesscss link