image.go make func encodedPNG() to EncodedPNG()

[zhuxiujia]

in image.go
make func encodedPNG() []byte to EncodedPNG() []byte,
it can support other http framework like "iris" "gin" !!!

and
1.
I recommend separate storage interfaces and image generator ，and delete storage interface，
For example,
com.google.code.kaptcha，
it only do the createText(),createImage().
other job like storage, i will choose like "redis" "mysql" "mongo"....

do not use "Id",and do not use "captcha.VerifyString(id string, digits string)" ,
I think the id just like session,but some people like me will use phone to Instead of id，and use
custom validation

[dchest]

• You can WriteTo image into the bytes buffer to get PNG []byte.

• You can use SetCustomStore to add a custom storage layer, including redis, mysql, mongo, or any other storage.

• ID is not a username or phone number, it's a captcha ID — random string. It is important for security. Use the ID returned by New, don't invent your custom ids, please.

[mxyue]

EncodedPNG() is useful