Clean up temporary file after CTRL+C
Fall back to notepad if the default editor is missing

If, for some reason, the application associated with text
files isn't available we'll just fall back to notepad.

Closes #14
Don't try to be clever. Only fall-back after-the-fact

Removed FindExecutable stuff in favor of catching the specific
"application not found" exception.
Don't be picky, fall back to notepad no matter what

Talked to @xpaulbettsx about this and his reasoning is that we want
GitPad to "just work"(TM) no matter what so we'll fallback to notepad
regardless of why Process.Start fails.

Also, don't confuse anyone by dumping the error into the console, just
make it work.
Merge pull request #15 from github/editor-hardening

Gracefully handle missing associations and editors