Scheme interpreter from tutorial

I added test-cases, implemented reader's exercises and polished code a little bit

Compatibility can be tested with online Scheme interpreter

Stack - the build system

Project was created by

cabal init - creates initial cabal config file
stack init - creates new stack project, stack.yaml etc.

Selfexplanatory

stack build
stack exec schemeinterpreter - this runs scheme repl :-)
stack test
stack repl - opens haskell repl