Remove unused import from backend.android.register.py

Testing Done:
ci.sh ran successfully.

Reviewed at https://rbcommons.com/s/twitter/r/589/
doc doc.py

a page about 'page' and 'goal markdown'

gave src/java/examples/hello a README.md and a corresponding BUILD target

tweaked a couple of doc pages that "included" hello/main/BUILD for a code example: tweaked them so that they wouldn't mention the page target; would be strange in that context.

Bugs closed: 257

Reviewed at https://rbcommons.com/s/twitter/r/582/
Fix missing search&replace of .buildfile -> .build_file

Find cases of .buildfile and replace with .build_file.  Running python tests with coverage was broken.

Testing Done:
Ran python tests with PANTS_PY_COVERAGE=1

Reviewed at https://rbcommons.com/s/twitter/r/593/
rm java/examples/pingpong and many of its 3rdparty deps

rm {src|tests}/java/com/pants/examples/pingpong

rm -r 3rdparty/jvm/com/

3rdparty/BUILD -= ['guice', 'easymock']

3rdparty/BUILD -= ['mockito'] <- pingpong didn't use this; 'git grep' says nothing else did either

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/28428093

Reviewed at https://rbcommons.com/s/twitter/r/596/
Payloads with sources now give more useful errors for nesting

Because the creation of the Payloads happens later in the process
(rather than during the build file parsing), we still seem to lose track
of which file the error occurred in... is there a solution for that?

- Adds validation to SourcesMixin and switches it to inheritance with
  SourcesPayload
- Adds a Payloads test file

Reviewed on https://rbcommons.com/s/twitter/r/586/
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).