Add simple variable assignment and definition

Only strings for now. Tests pending.
Merge branch 'master' into var-assignment

Conflicts:
	src/main/cup/ar/edu/itba/lang/Parser.cup
	src/main/java/ar/edu/itba/lang/ast/NodeType.java
	src/main/java/ar/edu/itba/lang/compiler/ASMVisitor.java
	src/main/jflex/ar/edu/itba/lang/Scanner.flex
Merge branch 'master' into var-assignment

Conflicts:
	src/main/java/ar/edu/itba/primer/compiler/ASMVisitor.java
Merge branch 'master' into var-assignment

Conflicts:
	src/main/java/ar/edu/itba/primer/compiler/ASMVisitor.java