Add a toString method to BytesRefArrayWritable

RCFileInputFormat cannot be used externally for Hadoop Streaming today cause Streaming generally relies on the K/V pairs to be able to emit text representations (via toString()).

Since BytesRefArrayWritable has no toString() methods, the usage of the RCFileInputFormat causes object representation prints which are not useful.

Also, unlike SequenceFiles, RCFiles store multiple "values" per row (i.e. an array), so its important to output them in a valid/parseable manner, as opposed to choosing a simple joining delimiter over the string representations of the inner elements.

I propose adding a standardised CSV formatting of the array data, such that users of Streaming can then parse the results in their own script. Since we have OpenCSV as a dependency already, we can make use of it for this purpose.