Only parse the current buffer if it was modified

A new buffer-local variable (fsharp-ac-last-parsed-ticks) is used the
keep track of BUFFER's tick counter at the moment when the buffer was
last parsed.
Make fsharp-ac-errors buffer-local in fsharp-mode buffers

The previous use of make-local-variable had no effect, because it made
the variable buffer-local only for current buffer at the time when
"fsharp-mode-completion.el" was loaded and not for future fsharp-mode
buffers.
fsharp-mode-wrapper: kill existing project buffers after test. refs #481
fsharp-ac/stop-process: Move cleanup code to process-sentinal function.

This will also make sure that the completion-process will get cleaned up
if it was killed/terminated outside Emacs.

Also make sure that errors get cleared in ALL fsharp buffers

Refs #481
Merge pull request #481 from juergenhoetzel/master

fsharp-mode-completion: Only parse the current buffer if it was modified