Lookup and Extend added.

Passes test 1 - 6.
Added set function.

Passes test 1-8 now. Also added docstring for
every function and reversed the 'if-statement'
in 'lookup' so that it's more logical.
Adapted evaluator.

Made the evaluator work with the environment. Passes all 13 tests of part 4.
Adapted evaluator to evaluate lambda.
Closure holds arguments given.
Lambda succesfully creates a closure

Passes test up to 7.
Added evaluation of closures.

Passes tests up to 10.
Added evaluation of closures.

Passes tests up to 10.
Added function calls.

Passes test up to 14 now. Had to take out the evaluate call
on the return of the is_symbol branch.
Error handling with functions.

Passes all tests from part 5.
Implemented lists and 'cons'.

Test 1 to 4 from part 6.
Added functions for lists.

Head, tail & empty. All tests of part 65 pass.
Added basic functions.
Last easy function added.

Passes tests up to 11.
Added functions on lists.

Passes tests up to 13.