Add base64encode and base64decode into builtin functions

[jiejie-dev]

Add # .

Changes proposed in this pull request:

• Add base64encode and base64decode into builtin functions

When one api required basic auth:

// This is a frank test case file
// -- https://github.com/txthinking/frank

url = "http://localhost:5000/v2.0"

POST /zone/login
header["Accept"] = "application/json"
header["Content-Type"] = "application/json"
header["Authorization"] = "Basic " + base64encode("jeremaihloo1024@gmail.com:admin")
json = {
    "email":"jeremaihloo1024@gmail.com",
	"password":"admin"
}

Response
must(status==200)

@txthinking Is there another way to do this instead of adding builtin functions ?

Sorry for my chinglish and bad PR comments.

[txthinking]

It looks like you missed an error checking

[jiejie-dev]

I have copied the error checking bellow.

@txthinking

[txthinking]

LGTM