DOC allow link embedding to proceed when web sites unavailable
Merge pull request #3571 from jnothman/fix-link-embedding

[MRG] DOC allow link embedding when some sites not available
MAINT CI: collect rackspace DNS info

Sometimes the following installation step fails:

    pip install -r continuous_integration/appveyor/requirements.txt

This steps download prebuilt version of numpy and scipy for windows that
are hosted on a rackspace container. In order to be able to report
the necessary debugging info to the rackspace cloud support team we need
to know the location of the failing hosts.
MAINT try to avoid HTTP timeouts to rackspace
Update about page
Remove unused distance calculation from clustering example.
DOC link to NLTK website

[ci skip]