feat: add support for md5-digest sasl auth.

[linsite]

Hello, I'm working with go-zookeeper, but found that SASL was not supported yet, as this issue says. As this feature is so import to my work that I do a little work and test, I think it works. Would you please take some time to reivew it? If there are problems I will try to fix them. Thanks for you time!

@jeffbean

[codecov]

Codecov Report

Merging #43 (28dff5b) into master (54f4812) will decrease coverage by 0.28%.
The diff coverage is 72.51%.

❗ Current head 28dff5b differs from pull request most recent head 1dd7138. Consider uploading reports for the commit 1dd7138 to get more accurate results

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
- Coverage   76.11%   75.83%   -0.29%     
==========================================
  Files           7        8       +1     
  Lines        1189     1295     +106     
==========================================
+ Hits          905      982      +77     
- Misses        195      214      +19     
- Partials       89       99      +10     

Impacted Files	Coverage Δ
constants.go	75.00% <ø> (ø)
structs.go	80.08% <0.00%> (-0.68%)	⬇️
conn.go	71.63% <56.92%> (-1.52%)	⬇️
sasl.go	90.47% <90.47%> (ø)
util.go	93.75% <100.00%> (+0.13%)	⬆️

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