[SPARK-6291] [MLLIB] GLM toString & toDebugString

GLM toString prints out intercept, numFeatures.
For LogisticRegression and SVM model, toString also prints out numClasses, threshold.
GLM toDebugString prints out the whole weights, intercept.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #5038 from yanboliang/spark-6291 and squashes the following commits:

2f578b0 [Yanbo Liang] code format
78b33f2 [Yanbo Liang] fix typos
1e8a023 [Yanbo Liang] GLM toString & toDebugString