Add 'encoding' parameter to override platform default

Following the guidelines at http://docs.codehaus.org/display/MAVENUSER/POM+Element+for+Source+File+Encoding
Use Maven plugin annotations for the mojo.

Prefer Maven plugin annotations for some measure of static
type safety. Add the necessary dependencies and plugins to
build and then use them.
Mark the mojo as thread-safe to avoid warnings in parallel builds.

There are no apparent issues in the plugin; mark it as thread-safe
under the assumption that Scalariform itself is thread-safe.