BENTO-29. Add kiji-express to bento box.

This commit does the following:
1. Modifies the pom.xml of bento-box to depend on kiji-express and
the express-music tutorial.
2. Ensures kiji-express and kiji-express-music are included in the
bento box distribution.
3. Modifies the README appropriately.
4. Modifies bento-version.txt appropriately.
5. Modifies kiji-env.sh to set EXPRESS_HOME and to add the express
command to the path.

Reviewed by: robertchu