add initial autotools support

Add support for generating Makefiles and configure scripts with
autotools. To use, run the following commands:

1. ./autogen.sh
2. ./configure
3. make

Rename the current Makefile to Makefile.scons so autotools won't overwrite
it. Makefiles that run scons are still present in subdirectories.

The autotools support builds libriak_c_client, riakc_example, and
tests. Also "make check", "make install", and "make distcheck" work.

Add generated protobuf files, generated release tarballs and a few other
files to .gitignore. Add generated protobuf files and src, examples, and
test subdirectories to distribution.