fixes #1773

Moved file and path related methods from StringHelper to FileHelper, renamed StringHelper byte methods not to be misused as string methods

as suggested in issue #1316

s/value to be cached value/value to be cached/g

Moved add prefix ($this->keyPrefix) to the key in function buildKey(). Could it be better?

Fixed an example of using buildKey in the doc

Appended newlines to every PHP document that did not have a newline as the last character in the file.