Introduce Cake.Issues.Build, for build server integrations.

Cake.Issues.Build should be used by build server addins in the future instead of Cake.Issues.PullRequests, which is a confusing naming.

Cake.Issues.Build shoudl only contain the functionality of Cake.Issues.PullRequests which makes sense for build servers:

• Settings
  • MaxIssuesToPost
  • MaxIssuesToPostForEachIssueProvider
  • ProviderIssueLimits, but as Dictionary<string, int>, since only MaxIssuesToPost is of interest
  • IssueFilters
• Base class: Similar to BasePullRequestSystem, but without capabilities
• ReportIssuesToBuildServer aliases

Idealy code used for pull requests and build servers is shared somehow (e.g. issue filtering).

Additionally documentation should be updated, everywhere where pull request systems where handled.