turned ProtobufCFieldDescriptor->packed into ->flags
minor adjustments to the field flags patch

rename PROTOBUF_C_FIELD_FLAGS_PACKED to PROTOBUF_C_FIELD_FLAG_PACKED.

rename ProtobufCFieldFlagType to ProtobufCFieldFlag.

wrap some particular long lines.

update documentation.

for clarity, use a "uint32_t" instead of "unsigned" for the 'flags'
field in _ProtobufCFieldDescriptor.
added deprecated flag into ProtobufCFieldDescriptor->flags

[edmonds@fsi.io: resolve merge conflicts, add documentation]
t/: add test case test_field_flags
ChangeLog: credit Andrei Nigmatulin for #138
rework header/library versioning

this replaces the changes in Issue #53 with a slightly different way of
representing / retrieving the version number.

protobuf_c_version() returns the version of the *library* as a string.

protobuf_c_version_number() returns the version of the *library* as an
integer.

PROTOBUF_C_VERSION is the version of the *headers* as a string constant.

PROTOBUF_C_VERSION_NUMBER is the version of the *headers* as an integer.
t/: add new test program 'version'
ChangeLog: versioning updates related to Issue #53
tightly couple protoc-c output with the protobuf-c headers

this adds a version guard like the protobuf C++ implementation. it
ensures that protoc-c and <protobuf-c.h> are from the exact same version
of protobuf-c.
ChangeLog: protoc-c/protobuf-c.h version coupling
ProtobufCFieldFlagType -> ProtobufCFieldFlag

this was inadvertently reverted when i was editing e9627054.
Makefile.am: ship LICENSE and README.md in the tarball