Add HTTP post /currency/add, with signature check

...re/src/main/java/io/ucoin/client/core/model/Currency.java

...main/java/io/ucoin/client/core/service/CryptoService.java

...oin/client/core/service/search/AccessDeniedException.java

...lient/core/service/search/DuplicateCurrencyException.java

...va/io/ucoin/client/core/service/search/SearchService.java

.../java/io/ucoin/client/core/service/CryptoServiceTest.java

.../io/ucoin/client/core/technical/crypto/SecretBoxTest.java

.../io/ucoin/client/ui/service/rest/CurrencyRestService.java

...oin/client/ui/service/rest/InvalidSignatureException.java