Add methods for GHE endpoints

[fiahil]

Hello,
This PR adds methods for Github Enterprise endpoints.

The following are covered:

• Create organization
• Create User
• Delete User

I will be happy to add a few fix on this PR if necessary.

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[codecov]

Codecov Report

Merging #1214 into master will decrease coverage by 0.04%.
The diff coverage is 65.71%.

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
==========================================
- Coverage   73.39%   73.34%   -0.05%     
==========================================
  Files          84       86       +2     
  Lines        5949     5984      +35     
==========================================
+ Hits         4366     4389      +23     
- Misses        825      831       +6     
- Partials      758      764       +6

Impacted Files	Coverage Δ
github/admin_stats.go	100% <ø> (ø)	⬆️
github/admin_users.go	63.63% <63.63%> (ø)
github/admin_orgs.go	69.23% <69.23%> (ø)
github/users_emails.go	58.62% <0%> (ø)	⬆️

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[MaesterZ]

Ok for my commits being contributed to this project 👍

[gmlewis]

Thank you, contributors.
We need to make a few changes before this can be merged, please.

@willnorris - can you please check the status of the CLA with the 3 authors and advise if this PR can be accepted?

[gmlewis]

Thank you for the changes, @fiahil!
Just one more tweak, please, and then we should be good to go after we get a second LGTM and @willnorris reports back about the CLA status.

[gmlewis]

Thank you, @fiahil!
LGTM.

Awaiting second LGTM and CLA status report.

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[gmlewis]

Thank you, @gauntface!
Merging.