Implement Migration - Source Imports client

[ryangribble]

https://developer.github.com/v3/migration/source_imports/

Added to API docs approximately Feb 19 2016

The Source Import API lets you start an import from a Git, Subversion, Mercurial, or Team Foundation Server source repository. This is the same functionality as the GitHub Importer.

/migration/sourceimports/
 - PUT /repos/:owner/:repo/import
 - GET /repos/:owner/:repo/import
 - GET /repos/:owner/:repo/import/authors
 - PATCH /repos/:owner/:repo/import/authors/:author_id
 - DELETE /repos/:owner/:repo/import

Note this is a preview API and custom accept headers are required to activate it application/vnd.github.barred-rock-preview

[devkhan]

Since I'm already implementing the Migrations API, should I take this up after I am finished with that?

[ryangribble]

Certainly, but it's definitely best to get one over the line before working on the next.l :grinning: Infact, I haven't yet started on #1024 which would be a higher priority IMO, if you wanted to knock off some of that coding :stuck_out_tongue_winking_eye:

[devkhan]

@ryangribble Sure, I'll try to make it production-ready ASAP.

[ginuraju]

@ryangribble Hi Ryan, does octokit has an implementation for source import endpoint to migrate from other git repos(non GitHub ones) to GitHub

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