Added pb_decode_delimited_noinit

I added API function to receive delimited data without initizializing the target structure. This is useful for partial updates.

Nanopb has traditionally supported messages to be terminated with a zero tag.
However, this is not really standard protobuf behaviour, so it makes sense
to separate it into a different function. Because it is a breaking change,
it will happen in 0.4.0 release. But I add the functions here early so that
new code can start using them now.

Also changed the network_server example to use en/decode_delimited(), which
is the more common protobuf method of simple message framing.

Fix Travis-CI Build

* For some reason this was failing to find the python lib with the
  previous invocation.
* Seems that the newer version of scons fixes an old bug, discard old
  method.

* Upgrade to protobuf release 3.4.
* Use the prebuilt protoc binary to skip alot of compilation steps
* Use the smaller Python source file for building python packages

* Simpler tool provided by coreutils.

Add new RELPATH option to address issue #271

This should allow using relative paths when importing .proto files.

See https://github.com/nanopb/nanopb/commit/cc74b9f200176edc5524aa00ba45fa90a5e87d27 for discussion.

Export examples

bazel: Add WORKSPACE

A WORKSPACE file is required for bazel to build the project. The
conventional naming is similar to Java packages with all special
characters including dots replaced by underscores.

In extra/FindNanopb.cmake, add dependency for .options files

the .options file (if it exists), so that editing the .options file will
trigger a rebuild.

Similar memset() already existed for oneofs.

pb_check_proto3_default_value() recurses into submessages,
but it didn't handle other than singular fields correctly.
This caused it to sometimes skip submessages with only
repeated or oneof fields present.