added parser unit test environment

`source/test/parser` contains a few snippets of code that allow easy testing
of the parser. At the moment, a test that reproduces issue #10 is the only
thing present; but I expect this list to grow over time.
moved base64 and sqlite3 to thirdparty dir

This makes it more obvious which source files come from an external
source, also helping GitHub to estimate the project's language
distribution more evenly.