Fix generics in interfaces

Not all interfaces' generics were self bound - this impacts return types.
Make BigDecimalAssert extendable
Make BooleanAssert extendable
Make ByteAssert extendable
Make CharacterAssert extendable
Make ClassAssert extendable
Make DateAssert extendable
Make DoubleAssert extendable
Make FileAssert extendable
Make FloatAssert extendable
Make InputStreamAssert extendable
Make IntegerAssert extendable
Make ListAssert extendable
Make LongAssert extendable
Make MapAssert extendable
Make ObjectAssert extendable
Make ShortAssert extendable
Make ThrowableAssert extendable
#82 : remove Fest references
Remove unused imports