Merge pull request #477 from sunpy/staging

Time to sync master with staging again
Fix behaviour when DummyAttrs remain in query.
Fix missing EOL.
Remove DummyAttr from HEK walker.
Add tests for HEK.
Make spectrogram plotting consistent.

Lacks updated docs!
Add download progress bar.
Fix downloader deadlock.
Fix refactor regression.

clip_freqs no longer updated the freq axis.
Update docs.
Fix tests.
Improve tests
Add a word about Anaconda.
Add word about multiple Python envs.
No longer leak fds.
Fix bug in downloader.
Improve test.

Add timeout to download test.
Code style change.
Remove test code.
Improve download test suite.