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