Add a leading 0 to binary data in fromBytes() constructor if most significant bit set (otherwise, the magnitude is interpreted as negative and this constructor fails)

[adam-singer]

https://github.com/financeCoding/dart-bignum/pull/1

[stevenroose]

This change is still only in the dev-version of the package, although is keeps the main release non-conform with the Java implementation.

Please update to 0.0.5 with this change included.