v2.4.0

What's Changed

• add payment request/response by @ilya-korotya in #48
• use string type by @ilya-korotya in #49
• add currency field by @ilya-korotya in #50
• Add build tag no_jwz to get rid of dependency on librapidsnark.a if you don't need it. by @olomix in #51

Full Changelog: v2.3.3...v2.4.0