Merge pull request #13 from rafacm/functions

functions: first cut
Merge pull request #14 from rafacm/functions

functions: updated documentation
Fixed FunctionSuite, it broke the build before and compiler was not able to compile tests
finalized pattern matching exercises
fixed link for NullExerciseSuite.scala
fixed typo in CollectionsSuite
TDD steps example
FIX - was giving away the solution
Improved comments
functions: last touches (issue #4)