Added picojson header only library.

Given that the make install was recently merged into that project but no release has been done yet, this is currently using a commit hash specific tarball from out of picojson's master branch.
It should get updated as soon as there is a proper release of picojson that includes the make install.

Can a test be added to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!