Add a means to configure how Flycheck displays error messages
Make error display more generic

The error display function now takes a list of errors to display.  The
corresponding variable is now `flycheck-display-errors-function'.
Update the change log

Also document the package version in the variable.
Document `flycheck-display-errors-function`
Add Robert Dallas Gray to credits

Thanks, @rdallasgray, for your contribution!
Fix test cases for `flycheck-display-errors-function`
Remove usage of non-existing functions

As a side-effect, copy each messages separately in
`flycheck-copy-messages-as-kill`.
[Fix #156] Merge branch 'configurable-display-function'

* rdallasgray-configurable-display-function:
  Remove usage of non-existing functions
  Fix test cases for `flycheck-display-errors-function`
  Add Robert Dallas Gray to credits
  Document `flycheck-display-errors-function`
  Update the change log
  Make error display more generic
  Add a means to configure how Flycheck displays error messages