Implement HTTP backend. Fixes #48

[osma]

No description provided.

[codecov]

Codecov Report

Merging #49 into master will increase coverage by 0.61%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   90.77%   91.39%   +0.61%     
==========================================
  Files          19       21       +2     
  Lines         347      372      +25     
==========================================
+ Hits          315      340      +25     
  Misses         32       32

Impacted Files	Coverage Δ
annif/backend/http.py	100% <100%> (ø)
annif/backend/__init__.py	93.54% <100%> (+0.44%)	⬆️
tests/test_backend_http.py	100% <100%> (ø)

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[coveralls]

Coverage increased (+0.6%) to 91.398% when pulling 73eb00c on http-backend into a13a086 on master.