Implemented Get / IsMember for teams

Issue #331.
Added integration tests and a new attribute that requires a new environment variable with an organization to test.

Conflicts:
	Octokit.Tests.Integration/Octokit.Tests.Integration.csproj
Update documentation on how to run the integration tests.
Fix build errors from renamed ITeamsClient members.
Added missing reactive members.
Fixed code analysis issue with naming of Get
Added missing check for empty/null login and its tests
Merge pull request #449 from kzu/GetAndIsMember

Implemented Get / IsMember for teams