[SPARK-7752] [MLLIB] Use lowercase letters for NaiveBayes.modelType

to be consistent with other string names in MLlib. This PR also updates the implementation to use vals instead of hardcoded strings. jkbradley leahmcguire

Author: Xiangrui Meng <meng@databricks.com>

Closes #6277 from mengxr/SPARK-7752 and squashes the following commits:

f38b662 [Xiangrui Meng] add another case _ back in test
ae5c66a [Xiangrui Meng] model type -> modelType
711d1c6 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-7752
40ae53e [Xiangrui Meng] fix Java test suite
264a814 [Xiangrui Meng] add case _ back
3c456a8 [Xiangrui Meng] update NB user guide
17bba53 [Xiangrui Meng] update naive Bayes to use lowercase model type strings