HEK2VSO and HELIO

This is all of @mjm159 's GSOC code.

ToDo:
* [x] Correct imports: https://github.com/sunpy/sunpy/pull/572#discussion-diff-6364379
* [x] Use existing download bar https://github.com/sunpy/sunpy/pull/572#discussion-diff-6367737
* [x] what is helioviwer.py doing here? https://github.com/sunpy/sunpy/pull/572#discussion-diff-6367802
* [x] Update documentation once #720 is merged.


Warning: Merge after #720