update changelog with the recent pull requests

Author: gasparnagy

changelog.txt

@@ -2,6 +2,9 @@
 
 Fixed issues:
 + Error when opening feature file in VS2008 if VS2008 syntax coloring was enabled.
++ Silverlight: make possible to access SilverlightTest instance (Issue 126, by Matt Ellis)
++ Silverlight: apply feature tags to the test class instead of the methods (Issue 126, by Matt Ellis)
++ SpecFlowContext.TryGetValue does not evaluate Func<T> factory methods (Issue 127, by Matt Ellis)
 
 1.7.1 - 2011/08/19