diff_match_patch.h is not self contained

```
What steps will reproduce the problem?
1. Create a simple program that just #include's diff_match_patch.h

What is the expected output? What do you see instead?
Should work. Instead, get lots of errors due to classes that are not defined.

What version of the product are you using? On what operating system?
SVN r103, on Linux.

Please provide any additional information below.
Please change diff_match_patch.cpp to include diff_match_patch.h before any
other headers and fix the resulting errors. This will ensure that
diff_match_patch.h is self contained (and is in general good programming
practice).

```

Original issue reported on code.google.com by `matthew....@kitware.com` on 8 May 2012 at 6:30

Inizia con diff_match_patch.cpp e diff_match_patch.h. Crea il programma minimo che includa solo diff_match_patch.h, quindi includi per primo l’header in diff_match_patch.cpp e compila per far emergere gli errori risultanti. Il lavoro è completato quando l’header può essere incluso autonomamente senza errori relativi a classi non definite e il codice C++ esistente continua a compilare.

Scritto dal modello di indicizzazione a partire dal testo della issue.