Always return size for messages with OneOf fields

Use a maximum calculation if submessage sizes are unknown.

Use protoc plugin mode for source generation in CMake

This commit uses protoc using the nanopb plugin to generate source files
in a single step instead of two. A new variable 'NANOPB_OPTIONS' can be
used to pass nanopb options to protoc.

The directory of a generated proto file is always passed as a search
path for option files.

This fixes the calculation of max encoded size for proto files with
imports.

Explicitly handle all enum values in switch

Add nanopb_PROTOC_PATH variable to CMakeLists.txt

Require non-buggy version of cmake

See https://github.com/pytorch/pytorch/issues/4141 and https://github.com/gflags/gflags/issues/113 for context

Thanks to David Vescovi for the fix.

Fix #296 - install path of Python plugins with CMake.

No need of specifying the CMAKE_PREFIX_PATH to python when asking for
sysconfig.get_python_lib since everything is installed relative to
CMAKE_PREFIX_PATH anyway.

Fix negative number without 64bit

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