Adapt to changes in OpenML API, bump versions, add maxbytes feature

[jbrea]

This addresses #16 and #17.

In addition,

• I refactored the handling of GET requests to simplify code maintenance.
• I adapted to changes in the OpenML API
• I added the keyword argument maxbytes to the load function to support loading of the first few rows of datasets.
• I removed the keyword api_keys, which didn't have any effect so far unless making some functions throw errors (this is technically breaking).

[codecov-commenter]

Codecov Report

Merging #18 (ccaee38) into dev (f2daf08) will decrease coverage by 2.06%. The diff coverage is 34.37%.

@@            Coverage Diff             @@
##              dev      #18      +/-   ##
==========================================
- Coverage   34.53%   32.46%   -2.07%     
==========================================
  Files           1        1              
  Lines         139       77      -62     
==========================================
- Hits           48       25      -23     
+ Misses         91       52      -39     

Impacted Files	Coverage Δ
src/data.jl	32.46% <34.37%> (-2.07%)	:arrow_down:

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[ablaom]

Many thanks @jbrea !

@darenasc or @OkonSamuel Going on leave for a week. Any chance one of you could review this?

Hopefully I can then jump and tag a new (breaking release) with the new HTTP compat.

[jbrea]

Does anyone have the time to look at this within the next few days? I would like to use the updated version for the course that starts mid September.

[jbrea]

Breaking is only the removal of the keyword argument api_keys. However, this keyword argument never did anything, so I don't expect to break user code.

[jbrea]

Thanks, @ablaom!