New Rule: Base64

[CSenshi]

Rule Name: Base64

Rule Description: The field under validation must be a valid Base64 encoded

Rule Usage Example:

reqs = {"data" : "VmFsaWRhdG9yIEZUVyE="}
rule = {"data" : "base64"}
validate(reqs, rule) # True

reqs = {"data" : "Not Encoded"}
rule = {"data" : "base64"}
validate(reqs, rule) # False, It fails because...

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Before contributing please review RULES.md (check for duplication), also check CONTRIBUTING.md for more details :100:

[varunhc]

Pull request for this issue: #113