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

**Problem**

I installed the terminal-notifier using ruby gems which puts it at

    /usr/bin/terminal-notifier

while the current implementation expects it at

    /usr/local/bin/terminal-notifier

I suggest it to use the OS `$PATH` env variable to find executables.

**Solution in Pull Request**

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.

Can also be configured with explicit search paths (i.e. no PATH env):

    // $PATH by default
    ExecutableLocator locator = new ExecutableLocator();
    // finding command
    if (locator.isAvailable("terminal-notifier")) {
        String absPath = locator.getPath("terminal-notifier");
        Runtime.exec(absPath);
    }

    // mixed search path referencing $PATH
    locator = new ExecutableLocator(new String[] { "/some/custom/bin", ExecutableLocator.PATH_ENV });

    // alternative way of adding search path step by step
    locator.clear();
    locator.add("/some/custom/bin");
    locator.add(ExecutableLocator.PATH_ENV);