New feature: Added sign() method to Math\BigInteger adapters

The sign() method is usefull to get the sign information about
a big integer. This can be also achieved with the comparison method,
but it's more handy to use the shortcut sign() method.
This is very common in other languages:
Java: http://docs.oracle.com/javase/6/docs/api/java/math/BigInteger.html#signum%28%29
C#: http://msdn.microsoft.com/en-us/library/system.numerics.biginteger.sign.aspx
Go: http://golang.org/pkg/math/big/#Int.Sign