Rename the error message popup buffer

Frees the name `*Flycheck errors*` for the new listing buffer
Add function to format an error as string
Work around dash bug

s-lex-format from dash.el fails, if the replacement text contains a
literal backslash, see magnars/s.el#34
Add a command to determine the Flycheck version
Track the checker that reported an error
Add `flycheck-error-format` to the manual
Add `flycheck-list-errors` to list all errors
Fix compilation warning
Merge branch 'error-listing'

* error-listing:
  Fix compilation warning
  Add `flycheck-list-errors` to list all errors
  Add `flycheck-error-format` to the manual
  Track the checker that reported an error
  Add a command to determine the Flycheck version
  Work around dash bug
  Add function to format an error as string
  Rename the error message popup buffer
Release Flycheck 0.12
Bump version in master
Bind `flycheck-version` to a key
Fix next link in changelog
Update the screenshot

The new screenshot uses a cooler font and demonstrates the new error
list feature.
Update the list of features in README/manual