Workflow improvements

@leggetter can you take a look at this and let me know if there are any extra improvements that need to be made before Tuesday?

- BRJS path is now taken as a property for the init task
  - e.g. `./gradlew init -Pbrjs=/some/path/to/sdk`
- now checks that the path supplied for BRJS is valid
- new task (`copyToBrjs`) to copy the jar in to brjs conf/java dir
- correctly generates the eclipse/idea classpath, unfortunately this has to be done as a separate run of gradle. So the workflow is now `gradle init` followed by `gradle eclipse`
- now adds the src jar to the classpath (for Eclipse only)