Return values now work; function args need to be checked.
Finally refactored Value class to have better method names (e.g.,
getIntValue() instead of getiValue()).  Also added a convertTo() to
Value to change between different data types.  That allowed me to change
PrintStatement so it can print any data type, not just strings.
Here's a better idea for type conversions: do them implicitly in the
calls to Value.getIntValue(), Value.getStringValue(), etc.  It's more
succinct and it avoids an unnecessary object copy since it returns the
value directly rather than a new Value.
Merge branch 'master' of https://github.com/pwholmes/geppetto