Add GitHub pull request support

[Jakub-Vacek]

Add method getPullRequests to class GitHubService with parameters: config: Config and pullRequestsCount: Int. This method will return array of PullRequest objects containing attributes:

• id: Int
• name: String - name of pull request
• url: String - web url to pull request
• state: Enum - state of pull request inProgress or merged or closed
• cratedAt: String - datetime string of pull request creation
• lastActivityAt: String - datetime string of pull request last activity (including merging or closing)
  In this array will be objects with newest lastActivityAt parameter and length of array is equal to parameter pullRequestsCount.

During implementation you can use GilHub api documentation
Depends on #1