GEOMESA-302 Export overwrites files without prompting

Exported files are now guaranteed to have a unique filename through using File.createTempFile,
which appends a unique id to each file name. The current DateTime is also appended
to make it easier to identify exported files.
An example is "geomesa_catalog_twittersmall_2014-08-19T15:08:29.725Z_2378129177436057594.tsv"