[SPARK-5596] [mllib] ML model import/export for GLMs, NaiveBayes

This is a PR for Parquet-based model import/export.  Please see the design doc on [the JIRA](https://issues.apache.org/jira/browse/SPARK-4587).

Note: This includes only a subset of regression and classification models:
* NaiveBayes, SVM, LogisticRegression
* LinearRegression, RidgeRegression, Lasso

Follow-up PRs will cover other models.

Sketch of current contents:
* New traits: Saveable, Loader
* Implementations for some algorithms
* Also: Added LogisticRegressionModel.getThreshold method (so that unit test could check the threshold)

CC: mengxr  selvinsource

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4233 from jkbradley/ml-import-export and squashes the following commits:

87c4eb8 [Joseph K. Bradley] small cleanups
12d9059 [Joseph K. Bradley] Many cleanups after code review.  Major changes: Storing numFeatures, numClasses in model metadata. Improvements to unit tests
b4ee064 [Joseph K. Bradley] Reorganized save/load for regression and classification.  Renamed concepts to Saveable, Loader
a34aef5 [Joseph K. Bradley] Merge remote-tracking branch 'upstream/master' into ml-import-export
ee99228 [Joseph K. Bradley] scala style fix
79675d5 [Joseph K. Bradley] cleanups in LogisticRegression after rebasing after multinomial PR
d1e5882 [Joseph K. Bradley] organized imports
2935963 [Joseph K. Bradley] Added save/load and tests for most classification and regression models
c495dba [Joseph K. Bradley] made version for model import/export local to each model
1496852 [Joseph K. Bradley] Added save/load for NaiveBayes
8d46386 [Joseph K. Bradley] Added save/load to NaiveBayes
1577d70 [Joseph K. Bradley] fixed issues after rebasing on master (DataFrame patch)
64914a3 [Joseph K. Bradley] added getThreshold to SVMModel
b1fc5ec [Joseph K. Bradley] small cleanups
418ba1b [Joseph K. Bradley] Added save, load to mllib.classification.LogisticRegressionModel, plus test suite