includes pairwise.adonis2 for strata

This is a major release. The function pairwise.adonis was reformulated to be used without strata and without interactions. Also degrees of freedom and sums of squares are returned now.
For the use of strata and interactions use pairwise.adonis2. This accepts formula as input and should be able to handle any model that adonis can handle.

some warnings excluded

Merge pull request #6 from jarioksa/if-length

use length-1 conditions in if()

v.02

• input can now be data frame or distance matrix
• examples to use strata are given

initial version of pairwise.adonis by pmartinezarbizu

This is the initial code of pmartinezarbizu of pairwise.adonis before merging the additions of smonteux