Allow the flycheck error parser to work on code issues and QuickFixes

It used to only work on QuickFixes.
Add flycheck checker for code issues (refactoring suggestions)
Re-add "Start syntax check" to the menu
Make flycheck checkers use shared code for getting the curl command
Attempt to chain syntax errors -> code issues checkers

Doesn't work yet, but I can't figure out why not.
Finish chaining!
Allow fixing the first code issue on the current line

There is a new example mapping for this (for evil-mode): g f
this lets the OmniSharpServer fix the issue and inserts the new contents
returned by the server. The effect is instantaneous!
Don't force the user to use both flycheck checkers in order

The user may want to use either one or the other, or use them in a
different order (okay, maybe not). Flycheck provides ways of customizing
and ordering the provided checkers, and this should not be forced on the
user by omnisharp-emacs.
Remove an unused function

This was created a long time ago, before the OmniSharpServer had ways of
managing path conversions. I don't think it was even ever used.
Merge branch 'feature-code-issues' into develop