GEOMESA 325: allow ingest without date information

* date information flags like dtformat and dtfield are now optional.
* added tests to check that writing the features worked.
* updated readme.
for future reference, check out the [Try](http://danielwestheide.com/blog/2012/12/26/the-neophytes-guide-to-scala-part-6-error-handling-with-try.html) monad.