Version 2.8.1

This is a maintenance release for poppr.

BUG FIX

• An error that appeared in some AMOVA calls with genind objects with character-
  based alleles was fixed (see #190 for details)

DOCUMENTATION

• aboot() documentation was updated to add the citation and make clear its
  purpose and limitations.

MISC

• DOIs have been hyperlinked to doi.org instead of dx.doi.org (#188, @katrinleinweber)