Add constructor for BigNumber instance.
Merge branch 'master' into prv-kkowalczuk-constructor

Conflicts:
	Block.cpp
	Block.h
int64_t -> uint64_t
Implement toString() method
Merge branch 'prv-kkowalczuk-constructor' of https://github.com/kkowalczuk/bignum into prv-kkowalczuk-constructor
Finished work with constructor & toString()