DDLSHELL-45. Support extended environment attrs for modules.

Adds EnvironmentPlugin SPI.
Modifies Environment and DDLCommand to track per-module state.
Adds unit tests.
DDLSHELL-43. Add USE JAR INFILE... command.

The Environment retains a list of 'libJars' that specify jar resources that
should be made available to subprocesses.
DDLSHELL-44. Support Java subprocess execution.

Adds the ForkJvm trait to assist DDLCommand implementations in
launching Java subprocesses.

Adds the 'fork' plugin module with the FORK JVM 'com.example.MyMain' command.
DDLSHELL-47. Plugin modules can declare help text.

Also incorporates DDLSHELL-36. (Incorrect link to schema shell user guide.)
Style improvements from previous commits.

Fixes some code style issues raised in:
  DDLSHELL-45. https://review.kiji.org/r/845/
  DDLSHELL-43. https://review.kiji.org/r/846/
  DDLSHELL-44. https://review.kiji.org/r/847/
  DDLSHELL-47. https://review.kiji.org/r/848/