DONE
-fix caching issue for compiler interface
-fix test ClassLoader


Goals/Guidelines for xsbt
=====
As usual:
 - Immutable interfaces
 - Typesafe
 - Robust, flexible API

ISSUES
 - Scala issue #2265: cannot use structural types or libraries that use structural types, or OOME: PermGen will occur.  Running/testing user code with structural types might also cause OOME, but is unlikely unless the Scala standard library or compiler uses structural types.

TODO
 - launcher interface versioning
 - allow comments in datatype definition file
 - tests for API extractor

update 0_6_Summary to mention different output directories.
look at permgen in Josh's ScalaTest
TrapExit handles System.exit(0) as nonzero exit code

Task engine
- method tasks will be normal tasks that pull the command line from a CommandLine task
- per task configuration, including logging (e.g. 'task compile log debug')
- unnamed tasks log to parent task 
- better control over parallel logging.  Approach undecided. Some possibilities:
  + optionally one task always logging
  + log each task to a file
  + interactive control over logging (might require tasks that use console to be explicit in their dependence on the console)

- Interfaces subproject that depends on no other project and defines interfaces in package xsbti.  They are written in Java and cannot refer to Scala classes (compileOrder = JavaThenScala).  These interfaces can be used to pass objects across ClassLoader boundaries.
- launcher/main interface is not static (no system properties should be used past the static entry point)
- simple, well-defined ClassLoaders
- use Exceptions instead of Option/Either
- every component gets its own subproject
- can use any version of compiler/Scala that is source compatible with the compiler-interface component
- build xsbt using normal cross-build conventions
- compiler: raw interface (no dependency analysis, but still in same jvm) or with dependency analysis
- compiler: can specify scala-library.jar and scala-compiler.jar + version instead of retrieving the ClassLoader
- compiler: new phase that extracts API defined in sources (classes, their members, and all type information)
- minimal dependence on main xsbt logger from subcomponents: use thin interface for subcomponents

Dependency Management
- resolvers are completely defined in project definition (use Resolver.withDefaultResolvers)
- configurations completely defined within project (use ModuleConfiguration.configurations)

Launcher
- see launch.specification
- boot interface contains static final int version = N that main xsbt can use to check if it can be loaded by that version (a lower bound check)
- main xsbt has static final int version = N that boot can use to check if it can load that version (a lower bound check)