Add GitHub API to create a pull request.

[yazeedobaid]

Description

This PR adds a new API to GitHub module to create a pull request on a repository.
The API usage is as follows:

let pullRequest = new PullRequest("Bump FAKE runner version", "version-branch", "master")

GitHub.createClientWithToken token
|> GitHub.createPullRequest "fsprojects" "fake" pullRequest
|> Async.RunSynchronously

Also, in the FAKE's build script, this API is used to open a new branch with the FAKE runner version bumped to the newest one after a release. Before this, the build script pushes directly to the master branch. However, this is no longer possible since the master branch is now protected and needs a PR when merging changes to it.