Add message stream resource

[tgrosinger]

Adding the message stream endpoints.

[codecov-commenter]

Codecov Report

Merging #35 (1d49bf7) into master (ef255e2) will decrease coverage by 0.53%.
The diff coverage is 92.53%.

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@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   97.15%   96.63%   -0.53%     
==========================================
  Files          12       13       +1     
  Lines         528      595      +67     
==========================================
+ Hits          513      575      +62     
- Misses         10       14       +4     
- Partials        5        6       +1     

Flag	Coverage Δ
unittests	96.63% <92.53%> (-0.53%)	⬇️

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Files Changed	Coverage Δ
postmark.go	72.72% <62.50%> (-1.75%)	⬇️
message_streams.go	96.61% <96.61%> (ø)

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[mrz1836]

LGTM