Add Lwan tests.

This patch adds Lwan web server alongside a benchmark that implements
all but one (database updates) tests.
I see nothing wrong with the code here, but you may visually prefer:  

    cmake $IROOT/lwan -DCMAKE_BUILD_TYPE=Release && make techempower'

Also, some people use a `bash_profile.sh` file to declare a new variable, like so: 

    #!/bin/bash
    export LWAN_ROOT=$IROOT/lwan

which can then be used inside your setup.py as so: 

    cmake $LWAN_ROOT -DCMAKE_BUILD_TYPE=Release && make techempower'

Similarly for your variable `build_dir`