Add methods to fetch gist's commits and specific revision

[fkling]

First of all, thank you for this library!

I need to be able to get a specific revision of gist, so I added support for these two API methods:

• https://developer.github.com/v3/gists/#list-gist-commits
• https://developer.github.com/v3/gists/#get-a-specific-revision-of-a-gist

[codecov-io]

Current coverage is 91.06% (diff: 100%)

Merging #412 into master will increase coverage by 0.29%

@@             master       #412   diff @@
==========================================
  Files            12         12          
  Lines           401        403     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            364        367     +3   
+ Misses           37         36     -1   
  Partials          0          0          

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[clayreimann]

Thanks for the contribution.

[fkling]

Thanks for merging so quickly!