Added asynchronous close method and updated documentation

[alexgolec]

No description provided.

[codecov]

Codecov Report

Merging #211 (8a080cf) into master (b4c62d2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          17       17           
  Lines        1958     1960    +2     
  Branches      172      172           
=======================================
+ Hits         1949     1951    +2     
  Misses          4        4           
  Partials        5        5           

Flag	Coverage Δ
unittests	99.54% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
tda/client/asynchronous.py	100.00% <100.00%> (ø)

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