Updates for demo

[bpoldrack]

• [x] Closes #84
• [x] allow to import DICOM tarball from URL
• [x] fix failure of extension's installation on Travis (fixed in datalad-webapp)
• [x] fix datalad-containers-list ( see https://github.com/datalad/datalad-container/pull/68 )

[bpoldrack]

FTR: Failure to register hirni extension actually is a datalad-webapp issue.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@c4be74e). Click here to learn what that means. The diff coverage is 91.66%.

@@            Coverage Diff            @@
##             master      #88   +/-   ##
=========================================
  Coverage          ?   75.27%           
=========================================
  Files             ?       13           
  Lines             ?      716           
  Branches          ?        0           
=========================================
  Hits              ?      539           
  Misses            ?      177           
  Partials          ?        0

Impacted Files	Coverage Δ
datalad_hirni/commands/spec4anything.py	33.69% <ø> (ø)
datalad_hirni/tests/test_dicom2spec.py	100% <100%> (ø)
datalad_hirni/tests/test_import.py	100% <100%> (ø)
datalad_hirni/tests/test_dicomconv.py	100% <100%> (ø)
datalad_hirni/commands/import_dicoms.py	92.47% <83.33%> (ø)

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