add initial autotools support

Add support for generating Makefiles and configure scripts with
autotools. To use:

1. run the ./autogen.sh script in top level directory
2. ./configure
3. make

Rename the current Makefile to Makefile.scons so autotools won't overwrite
it.

In this commit the autotools support builds only libriak_c_client and
riakc_example. It doesn't build any tests. Also "make distcheck" works,
which is critical for making sure release packages will build and install
properly.

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