Quit error message window [#94]

Quit window showing the error message buffer when navigating away from
error messages.
Update list of supported languages in introduction

[ci skip]
Add a Github ribbon referencing the repo

[ci skip]
Expand the copyright description a bit

[ci skip]
Fix ERT cross references in HTML manual
Move package meta data back to flycheck.el

Update Carton to 0.2 which is required for the `package-file` directive.
Bump dash dependency to latest version
Remove the generated -pkg.el
Properly quote shell variable
Move screenshot to doc/ directory

Keeps the root directory clean.

[ci skip]
Improve Make rules for HTML buildings

Really consider all sources, and reduce rebuilds
Add some informative frontmatter to HTML
Add screenshot to introduction in HTML
Improve some wording in README
Remove reference to Flymake

There is nothing particularly important in the Flymake manual that would
help Flycheck users.  Thus the prominent reference is misplaced, and
better fit in the credits.
Remove Github specific HTML stuff

This can easily be created on the gh-pages branch, and doesn't need to
be part of the build system.
Clear old Flycheck state when enabling the mode
Hide the error buffer when clearing [#94]
[Fix #105] Instrument flycheck-error-with-buffer
Restore alphabetic ordering of syntax checkers [#101]