Add a "require file checksum" rule

Hi.

Sometimes there is a need to have a rule to check a file against a predefined checksum. Let's say, a result of a generated file in the build output directory is expected to result in a required checksum, or some source/test files must result in predefined checksums. I've came across such a rule on GitHub https://github.com/esamson/checksum-enforcer-rule and I think it would be nice to have this one as a standard rule.

What I've done already:

• Adapted the original source to the source code base in http://svn.apache.org/repos/asf/maven/enforcer/trunk (1.4.2-SNAPSHOT)
• Added Apache Commons Codec 1.6 dependency thus adding support for MD5, SHA-1, SHA-256, SHA-384, SHA-512
• Added some tests
• Added an example for all hashing functions to the generated site.

What I'm not sure about:

• Would you accept this feature as one that's worth be a part of standard rules.
• If so, I'm not sure if https://github.com/apache/maven-enforcer accepts pull requests as I've noticed probably automatically generated `git-svn-id` field in `git log` output. So probably the patch might be committed to the Subversion repository first.

Here is the patch diff on GitHub: https://github.com/lyubomyr-shaydariv/maven-enforcer/commit/86ed97423b5c64625f700c3afe555c02d6c619df