add ASN.1 support (#40)

* add ASN.1 support

* add bytes mode for SM2 digital signature:   crypto/internal/sm2/sm2.go
add the corresponding test:   crypto/internal/sm2/sm2_test.go
add SingMode option:   crypto/keypair/sm2.go
add the corresponding test:   crypto/keypair/sm2_test.go
support bytes mode:   crypto/sm2/sign.go
support bytes mode:   crypto/sm2/verify.go

Author: Web-Coke