Refactor StringAssert into CharSequenceAssert so StringBuffer and StringBuilder could be used as well

[Crazyjavahacking]

I think that FEST assertions could refactor the StringAssert into CharSequenceAssert class. The idea is to support both StringBuilder and StringBuffer in a very natural way, without converting them to String which is less readable.

Internally, the CharSequenceAssert should have exactly the same code as StringAssert, just the conversion from CharSequence to String will be done internally. That means almost no changes in code.

Joes suggested, that we could still have a StringAssert class inheriting from CharSequenceAssert, so that StringAssert is still here and you could have StringBuilder and StringBuffer assertions.

Any other opinions on that change?