Make string2byte conversions indepedent of platform default encoding

Both the library itself and many of its tests are utterly dependent on the JVM's default charset. For example, DigestUtils calls String.getBytes() to convert an input string to a byte array, happily delivering different digests for the same input string if run on different platforms.

If you want to try out the havor yourself, just run the unit tests in a JVM with UTF-16, e.g. by adding the line

<argLine>-Dfile.encoding=UTF-16</argLine>

to the configuration of the Surefire Plugin in the POM.