functions support, tests, prettyprinter
if/while support
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/ru/spbau/jirkov/tralalang/AST.scala
	src/ru/spbau/jirkov/tralalang/Interpreter.scala
	src/ru/spbau/jirkov/tralalang/InterpreterTest.scala
	src/ru/spbau/jirkov/tralalang/ParserTests.scala
	src/ru/spbau/jirkov/tralalang/TralaParser.scala