Merge pull request #2 from lummax/fix_upcast_with_as_operator

Fix upcast with 'as' operator

Merged after 30 days.
Add travis-ci configuration
Merge pull request #6 from lummax/travisci_setup

Add travis-ci configuration
Set type of StringLiteral in ResolveVisitor
Implement (un)boxing of Strings
Test cases for String-constructor
Test cases for String as Object