Making sure that IOException is not thrown if target CSV file exists

Hi Coda,

When playing with Metrics (and using CsvReporter) I noticed that ```CsvReporter.createStreamForMetric()``` throws ```IOException``` when file cannot be created (for example it already exists). This ```IOException``` was flooding my logs and console, and since I had CsvReporter invoked on a minutely basis I got rather annoyed :)

I have added a check to the above method that returns ```PrintStream``` if the CSV file in the output directory exists and readable. In case if file is not there or not readable, exception is still thrown upon failed file create