fwdpp 0.5.7

This release has several API improvements. A big one is #55, which merges mutation and recombination into a single algorithm, improving performance in simulations where the probability that an offspring's copy of a region is both mutated and recombined is high.

Other changes:

• License change from GPL2 to GPL3
• The unit test suite is now compiled with warnings enabled. As a result, several warnings were silenced, including one
  introduced in PR55 that would have mattered!. PR63
• API to KTfwd::sample_diploid was updated to removed an unused type as a result of PR54. PR63
• Gametes can now be constructed from tuples. PR62
• KTfwd::popgenmut, KTfwd::generalmut, and KTfwd::generalmut_vec may now be constructed from tuples. PR59
• Travis build system now skips OS X builds, tries more GCC versions as well as C++11 and C++14.
  PR60 and PR61.
  We are skipping OS X because there have been stability problems with the Travis CI infrastructure for that platform.
  We would liked to have included Linux/clang tests for this release, but hit road blocks that we cannot reproduce on our
  local machines.
• Population objects from sugar layer can now be constructed with pre-calculated diploids, gametes, and mutations.
  PR56.
• Mutation and recombination are now merged into one path. See fwdpp/mutate_recombine.hpp. The entry points into the
  old API are marked deprecated. This addresses issue Issue 54 via pull request PR55.
• Travis builds using miniconda now use -Wl,-rpath when linking to dependencies, solving run-time link errors during "make check" on OS X.