Fix some edge cases. (#66)

If both range expressions are literal integers, ensure the lower bound is less
than the upper bound. Also, check for negative integers in either lower or upper
bound.

For expressions which are negative intgers, literal strings, literal regexp or
literal float are all now errors in gyp.

Fix incorrect parsing of multi-line comments in hex strings. (#58)

Closes #57.

Fix issue with identifiers with wildcards. (#47)

This fixes the same issue discussed in
VirusTotal/yara#1631.

Implement "defined" operator.