Add resolving compatibility for Sonatype Nexus 3 API

[joerg-wille]

API between Nexus 2 and 3 is not compatible. Also resolving maven artefacts does not work as expected (see open issue https://issues.sonatype.org/browse/NEXUS-12469). Therefore the newest asset is being returned as the latest snapshot.

The detection for Nexus 3 API is also done very crude, by checking for HTTP response for a "List repositories" get-request.

[stardust85]

Thanks for the contribution! And sorry for the big delay. The travis fails because of sphinx not compatible with python 2.6. @oskrkal, @drujd can we drop support for python 2.6? I don't need python 2.6 and I guess most of our users also don't. The same for Python 3.3

[stardust85]

This PR solves (at least partially) issue #25