Make spectrogram plotting consistent.

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

clip_freqs no longer updated the freq axis.
Update docs.
Fix tests.
Improve tests
Put ProgressBar into util. Document ProgressBar.
Enable sphinx doctest.
Disable doctests for reference/
Patch helioviewer test
Bugfix preventing download without progress bar to work.
Merge remote-tracking branch 'upstream/staging' into staging
Return axes.
Bugfix.
Merge pull request #501 from segfaulthunter/doctest

Test guides using doctests
Merge pull request #496 from segfaulthunter/dwprogressbar

Add download progress bar.
Merge pull request #494 from segfaulthunter/spectroplot

Make spectrogram plotting consistent.
Merge remote-tracking branch 'upstream/staging' into staging
Add Travis section to dev docs
Complete io tests.

Closes #217 and #216