Minor changes to allow compiling on unix

I needed a RethinkDB C++ driver for a quick project (more on that in separate cover), and found yours. I did have to make a few minor changes to get it working for me, and thought I should contribute them back. Nothing big, but I probably broke your settings in VisualStudio (I don't have that setup). The one change that might be controversial is the moving of your bootstrap `pro to` folder to `proto/rdb_protocol`. I did this to make compiling against the full RethinkDB source easier (what I am doing).