ChangeLog: fix formatting of author names
add code coverage option to the build system

configure now has a "--enable-code-coverage" option, and if the
lcov/gcov tools are available a code coverage report can be built with
"make check-code-coverage".

this is mostly based on the AX_CODE_COVERAGE / GNOME_CODE_COVERAGE
macro:

    http://savannah.gnu.org/patch/?8451

but i also looked at knot-dns's code coverage implementation:

    https://github.com/CZNIC-Labs/knot/blob/master/m4/code-coverage.m4

and kevin lyda's pull request:

    https://github.com/protobuf-c/protobuf-c/pull/131/files
.travis.yml: enable code coverage checks and upload data to coveralls.io
ChangeLog: --enable-code-coverage
README.md: add coveralls.io badge
protobuf-c/libprotobuf-c.pc.in: export the 'bindir' variable
loosen the coupling between protoc-c and the protobuf-c headers

exact version coupling between the compiler and the public headers is
too strict because some existing projects (such as collectd,
riemann-c-client, and nmsg) directly embed .pb-c.h files generated by
protoc-c into their exported headers. this would cause unnecessary build
failures in downstream clients of these libraries if a newer version of
the protobuf-c headers is installed.

however, it's still desireable to be able to explicitly declare when
compatibility is broken between the headers and the compiler, so
introduce new variables that allow independently setting the minimum
header version required by the compiler and the minimum compiler version
required by the header.

this follows the protobuf C++ implementation a little bit more closely,
though we don't have an analogous facility for verifying that the header
and *library* are compatible. (this seems like overkill for a C project;
in practice the headers and the library will be from the same version,
especially in downstream distributors like debian where the -dev package
has an exact versioned dependency on the shared library package.)
configure.ac: use the 5-argument form of AC_INIT() and also define our own PACKAGE_DESCRIPTION
Doxyfile.in: new

this is the completely unmodified output output from running
"doxygen -g Doxyfile.in".  (note that this is doxygen 1.8.7.)

customization will come in a subsequent commit so that we have a record
of exactly what was changed from the default values.
DoxygenLayout.xml: new

this is the unmodified output from "doxygen -l DoxygenLayout.xml".
Doxyfile.in: initial customizations
DoxygenLayout.xml: initial customizations
switch to a more automake-ish method of generating the html doxygen documentation
protobuf-c: __PROTOBUF_C_RUNTIME_H_ -> PROTOBUF_C_H

identifiers that begin with a double underscore are reserved.
_PROTOBUF_C_FORCE_ENUM_TO_BE_INT_SIZE -> PROTOBUF_C__FORCE_ENUM_TO_BE_INT_SIZE

identifiers that begin with an underscore are reserved. instead, use a
double underscore in the *middle* of the identifier to indicate that
it's an "internal" identifier.
Doxyfile.in: enable MACRO_EXPANSION

this avoids littering function definitions in the documentation with
"PROTOBUF_C_API".
remove PROTOBUF_C_ASSERT_NOT_REACHED from the public API

this macro isn't used anywhere except in the libprotobuf-c
implementation.
PROTOBUF_C_ASSERT -> assert

assert() conforms to the C89 standard, just call it directly.
use double underscores in the MAGIC identifiers

these magic constants aren't intended for use by client code. add a
double underscore to indicate this.
'struct _ProtobufC' -> 'struct ProtobufC'

identifiers in any scope that begin with an underscore and a capital
letter are reserved in C.