[Issue #43]: AppEngine REPL is broken

SessionBuilder was not respecting the FileSystemManager provided
by the caller and instead always instatiated the default fs manager,
which violates security permissions in the appengine environment.
Fixes several compile/build errors

Only noticed these when building from a fresh
machine...
Cleaned up SessionBuilder/RenjinScriptEngineFactory relationship

o Use SessionBuilder for specifying all parameters of the new
  Renjin Session
o The Session instance can be passed to RenjinScriptEngineFactory
  to get a ScriptEngine interface with the customized session

Should help resolve Issue #46