0.4.2 (#44)

• Adopt pipda 0.4.4
• Add varname to dependency to close #30
• Rename datar.datar_versions to datar.get_versions
• Port a set of functions from r-base, incluing:
  • prod, sign, signif, trunc, exp, log, log2, log10, log1p,
  • is_finite, is_infinite, is_nan,
  • match,
  • startswith, endswith, strtoi, chartr, tolower, toupper,
  • max_col