Add formula for eSpeak

Adds a formula for the [eSpeak](http://espeak.sourceforge.net) text-to-speech tool.
Upon investigation, eSpeak uses a `sem_timedwait` call that's not present in OS X, so `make install` fails with an error like this:

    $ brew install espeak
    ==> Downloading https://downloads.sourceforge.net/project/espeak/espeak/espeak-1.48/espeak-1.48.04-source.zip
    Already downloaded: /Library/Caches/Homebrew/espeak-1.48.04.zip
    ==> make install LIBDIR=/usr/local/Cellar/espeak/1.48.04/lib DATADIR=/usr/local/Cellar/espeak/1.48.04/share/espeak-data PREFIX=/usr/local/Cellar/espeak/1.48.04
    fifo.cpp:315:17: error: use of undeclared identifier 'sem_timedwait'
                    while ((err = sem_timedwait(&my_sem_start_is_required, &ts)) == -1
                                  ^
    1 error generated.
    make: *** [x_fifo.o] Error 1

I can look into patches to either implement `sem_timedwait` or just replace it with `sem_wait` and an `alarm`, but I don't think the library itself officially supports OS X. (The command works, just not the library).