Mac: doesn't find terminal-notifier if in different path

To solve it, added ExecutableLocator that searches the OS
PATH environment variable (by default) for executables.
Only used for the MacOSXNotifier so far, but other notifiers
could use it as well, it has support for windows exe's as well.
Merge pull request #13 from alexkli/master

Mac: doesn't find terminal-notifier if in different path