Implemented vector value in BigInteger

[1010jms]

• Added 'std::vector value' to BigInteger.hpp
• Added 'std::vector get_value' to BigInteger.hpp & BigInteger.cpp
• Added 'bool sign' to BigInteger.hpp
• Added 'bool get_sign' to BigInteger.hpp & BigInteger.cpp
• Modified 'std::string value' to 'std::string str_value' in BigInteger.hpp
• Modified default constructor to set 'value' to 0 and 'str_value' to "0" in BigInteger.cpp
• Modified copy constructor to account for vector 'value' in BigInteger.cpp
• Modified constructor 'BigInteger(std::string val)' to account for vector 'value' in BigInteger.cpp
• Added constructor 'BigInteger (std::vector val)' to BigInteger.hpp & BigInteger.cpp
• Added a plethora of test cases for the various constructors to unit-BigInteger.cpp

[1010jms]

Given/When/Then comments have been added to the test cases. Also, "test_really_really_big_numbers" was slightly modified to more accurately test the values of the third BigInteger (n3).