Add Lwan tests.

This patch adds Lwan web server alongside a benchmark that implements
all but one (database updates) tests.
Quick thought: Is there really any reason to expose `lwan.sh` as a separate dependency? If you don't expect another test to use `fw_depends lwan` anytime soon, it's better to just replace this line with the entire contents of you `lwan.sh`. The main benefit: if anything changes inside toolset/ then Travis-CI will re-run the *entire* test suite, but if only files in frameworks/C/lwan change then just lwan will be verified and that's like 100x faster.