admixr 0.3

• All wrappers have been given simpler names (qpDstat() -> d(),
  qpF4ratio() -> f4ratio(), etc).
• F4 statistic can now be calculated using a separate f4() function (f4mode
  parameter remains in the d() function though, as f4() calls d()
  internally).
• All tests are performed on Travis CI using installed and compiled ADMIXTOOLS
  software.
• Changed names of function arguments of qpAdm and output directory argument of all wrappers.