added classify cmd option

added classify cmd option; so that the `classify` examples from [tutorial](http://factorie.cs.umass.edu/tutorials/UsersGuide01Introduction.scala.html) could be executed, e.g.:

```
$ bin/fac classify --read-text-dirs sportsdir politicsdir --write-classifier mymodel.factorie
```
Merge pull request #125 from neil-rubens/patch-1

added classify cmd option