construction from hex strings

[waegemans]

Closes: #37 Changes:

• Allows construction from hex string (e.g. BigInt("0xaf123e"), BigInt("-0xff")). Hex string is converted to decimal during construction.
• Tests for construction from hex
• Remove unused bool is_valid_number(const std::string& num) in file include/functions/utility.hpp

[touatily]

Handling any other base would have been better