Update user agent string to include OS and Python version info

[mkistler]

Small update to add OS and Python version info to the user-agent string.

On my Mac, this results in:

{'user-agent': 'watson-apis-python-sdk-0.26.1 Darwin 16.7.0 2.7.12'}

In a docker container running the "python" image, this results in:

{'user-agent': 'watson-apis-python-sdk-0.26.1 Linux 4.9.49-moby 3.6.3'}

[codecov-io]

Codecov Report

Merging #275 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   85.22%   85.29%   +0.07%     
==========================================
  Files          24       24              
  Lines        1225     1231       +6     
==========================================
+ Hits         1044     1050       +6     
  Misses        181      181

Impacted Files	Coverage Δ
..._developer_cloud/watson_developer_cloud_service.py	76.96% <100%> (+0.69%)	⬆️

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[germanattanasio]

I hope we don't run into issues when running this into an Arduino. It's a Unix so it should work.