RateLimit class encapsulates rate limit headers

Added a RateLimit class to encapsulate pulling rate limit information
from the headers. This is now exposed by ApiInfo as well as the
RateLimitExceeededException. That way these two classes are not grabbing
the same information in different ways which we were doing before.

Octokit.Tests/Models/ReadOnlyPagedCollectionTests.cs

@@ -23,7 +23,7 @@ namespace Octokit.Tests.Models
                 var response = new ApiResponse<List<object>>
                 {
                     BodyAsObject = new List<object>(),
-                    ApiInfo = new ApiInfo(links, scopes, scopes, "etag", 100, 100)
+                    ApiInfo = new ApiInfo(links, scopes, scopes, "etag", new RateLimit(new Dictionary<string, string>()))
                 };
                 var connection = Substitute.For<IConnection>();
                 connection.GetAsync<List<object>>(nextPageUrl, null, null).Returns(nextPageResponse);