UanM: improve examples (see desc)

[pahp]

1. I'd like to roll "code injection" into Samy Worm (or vice versa) and mention XSS, SQLi, and pathname attacks.
2. Edit the section on Buffer Overflows
3. Add an example about input validation (not just with code injection)

[hijodecoche]

Rewrote the Code Injection section to include reflected XSS, persistent XSS, and SQL injection. Rewrote the section on buffer overflows, but someone should double check it. Added an example of input validation from the CVE database http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-5893