Allow setting base_url when instantiating the client

[richtier]

fixes this issue

[codecov]

Codecov Report

Merging #42 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   96.35%   96.41%   +0.06%     
==========================================
  Files          10       10              
  Lines         274      279       +5     
==========================================
+ Hits          264      269       +5     
  Misses         10       10

Impacted Files	Coverage Δ
alexa_client/alexa_client/client.py	95.45% <100%> (+0.21%)	⬆️
alexa_client/alexa_client/connection.py	97.36% <100%> (ø)	⬆️
alexa_client/alexa_client/constants.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 39f6b9e...8a9688f. Read the comment docs.