support for assertEquals([String message, ] CharSequence, CharSequence)

This contribution fixes a problem I've run into when using assertEquals on two CharSequence.

It basically already works, of course - BUT one is left wondering why e.g. the Eclipse (and presumably other IDE as well) JUnit view's Result Comparison pop-up does not work on double-clicking a failure.. this is because, currently, assertEquals on two CharSequence throws only a generic AssertionFailedError and not the ComparisonFailure expected by such UIs.

PS: As the pom.xml specifies jdk.version 1.5 and <compilerVersion>1.5</compilerVersion>,  I was assuming, hopefully correctly, that introducing a @since 1.4 CharSequence was not a problem for backward compat.
@vorburger it's always seemed strange to me that the assert methods are public static, but I support you are right that making these new ones protected would be inconsistent with what's already there. Let's change them back to public.

Once you do that, would you please squash the commits, as documented in https://github.com/junit-team/junit/wiki/Pull-Requests ?