Set up build system

[mattyohe]

With #29 merging in, we'll want to make sure that any new commits are still compilable Playgrounds.

• Set up circle project
• Make a build script that reports on the comparison output of generate_readme and the README.md in the project
• Compile sources to verify we're still producing compilable code.

@eriklamanna Does the generate_readme command just spit out the file? Does it halt if there's syntax issues?

[eriklamanna]

@mattyohe the generate_readme does not compile or run the playground. It's statically going through the .swift files and creating the top level markdown file.

[mattyohe]

@eriklamanna So we would likely just need to compile all the sources as part of a build script I imagine... Cool.