NS1 DNS Authenticator (#4602)

Implement an Authenticator which can fulfill a dns-01 challenge using
the NS1 DNS API. Applicable only for domains using NS1 DNS.

Testing Done:
 * `tox -e py27`
 * `tox -e lint`
 * Manual testing:
    * Used `certbot certonly --dns-nsone -d`, specifying a
      credentials file as a command line argument. Verified that a
      certificate was successfully obtained without user interaction.
    * Used `certbot certonly --dns-nsone -d`, without specifying a
      credentials file as a command line argument. Verified that the
      user was prompted and that a certificate was successfully
      obtained.
    * Used `certbot certonly -d`. Verified that the user was prompted for
      a credentials file after selecting dnsimple interactively and that
      a certificate was successfully obtained.
    * Used `certbot renew --force-renewal`. Verified that certificates
      were renewed without user interaction.
 * Negative testing:
    * Path to non-existent credentials file.
    * Credentials file with unsafe permissions (644).
    * Path to credentials file with an invalid token.
    * Path to credentials file without a token.
    * Domain name not registered to NS1 account.

certbot-dns-nsone/docs/Makefile

@@ -0,0 +1,20 @@
+# Minimal makefile for Sphinx documentation
+#
+
+# You can set these variables from the command line.
+SPHINXOPTS    =
+SPHINXBUILD   = sphinx-build
+SPHINXPROJ    = certbot-dns-nsone
+SOURCEDIR     = .
+BUILDDIR      = _build
+
+# Put it first so that "make" without argument is like "make help".
+help:
+	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+.PHONY: help Makefile
+
+# Catch-all target: route all unknown targets to Sphinx using the new
+# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+%: Makefile
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)