Allow readFile() in CakeEmail to be accessed publicly

I needed this method while building a Transport, and saw no reason why it needed to be protected.
Create method readAndBase64Encode() in File utility.

Move this utility method out of CakeEmail, which allows other Mail transports to encode files manually. Maintains BC.
Make readBase64 an instance method

Leverage the existing code in File::read() and simply add in chunking and base64 encoding.
Add to docblock the reason why the output is chunk_split
Update link url to revised method name
Change variable name to be descriptive
Remove readBase64() from File utility, since it is not commonly used.
Merge pull request #1284 from jmillerdesign/public-cakeemail-readfile

Utilize File utility to base64 encode email attachments