Made progress on environment and lookup
got variables working. still no support for trying to eval a variable that isn't in the environment
so many questions!!!
Variables now only consist of Strings. This solved scope issue.
addVar function added
added tests, depends on HUnit

cabal install hunit
Merge pull request #2 from spacemanaki/unit-tests

added tests, depends on HUnit
new test case for variables
let expression added along with test cases.
added CallF to eval along with test case.
eval now returns expr instead of int, allowing functions to remain functions when evaluated