Merge pull request #190 from rgsilva/master

Multiple variable declaration in one line
- Added check for already declared variable
- Added initial grammar for if-elseif-else control flow
- Initial code for packagemanager
- Initial code to import modules
- Make std.io.* working (calling its print and println funcs)
- Fixed memory leak on pkgmanager
- Comments, new methods, otimization...
- Simplify pkgmanager code
- Comments++
- Removed unused operand type
- Implemented while loop
- Changed OP_RET to work with no operand
- Changed VM::run() handler calling way
- Fixed tests
- Fixed OP_RET handler
- Removed OP_HALT and OP_PRINT
- Switch to doc comments
- Added string type
- Symbol is now a struct
- Renamed some Scope methods