More tests and minor things

• Some more patches to get the code coverage in digest/ to 98%.
• Now using DigestUtils.getDigest() to get rid of the checked exceptions.
• Removed superfluous long->int bit operation ("0xffffffff") as discussed on the mailing list
• Removed unnecessary "headerFile" line from pom.xml as it was moved to the <properties> tag.

The enhancements are available as several commits on:
https://github.com/lathspell/commons-codec/commits/trunk