Added support for including .proto files in remote .jars on the --proto_path when compiling with protoc.

This is done by way of a new 'imports' parameter added to the java_protobuf_library target, which accepts jar dependencies as inputs. The syntax for this is:

java_protobuf_library(name='foo',
  imports=jar_sources(jar(org='foo.com', name='bar', rev='1.2.3'), jar(org='bar.com', name='foo', rev='3.2.1')),
  ...,
)

An extra ivy step called 'ivy-imports' (rather than ivy-resolve) has been added to accomodate this. It occurs after bootstrap and before codegen. The build was failing for a while on my machine due unrecognized symbols generated by protoc; this was fixed by deleting ~/.pants.d and re-running (for reasons that are still opaque to me).