[SPARK-1636][MLLIB] Move main methods to examples

* `NaiveBayes` -> `SparseNaiveBayes`
* `KMeans` -> `DenseKMeans`
* `SVMWithSGD` and `LogisticRegerssionWithSGD` -> `BinaryClassification`
* `ALS` -> `MovieLensALS`
* `LinearRegressionWithSGD`, `LassoWithSGD`, and `RidgeRegressionWithSGD` -> `LinearRegression`
* `DecisionTree` -> `DecisionTreeRunner`

`scopt` is used for parsing command-line parameters. `scopt` has MIT license and it only depends on `scala-library`.

Example help message:

~~~
BinaryClassification: an example app for binary classification.
Usage: BinaryClassification [options] <input>

  --numIterations <value>
        number of iterations
  --stepSize <value>
        initial step size, default: 1.0
  --algorithm <value>
        algorithm (SVM,LR), default: LR
  --regType <value>
        regularization type (L1,L2), default: L2
  --regParam <value>
        regularization parameter, default: 0.1
  <input>
        input paths to labeled examples in LIBSVM format
~~~

Author: Xiangrui Meng <meng@databricks.com>

Closes #584 from mengxr/mllib-main and squashes the following commits:

7b58c60 [Xiangrui Meng] minor
6e35d7e [Xiangrui Meng] make imports explicit and fix code style
c6178c9 [Xiangrui Meng] update TS PCA/SVD to use new spark-submit
6acff75 [Xiangrui Meng] use scopt for DecisionTreeRunner
be86069 [Xiangrui Meng] use main instead of extending App
b3edf68 [Xiangrui Meng] move DecisionTree's main method to examples
8bfaa5a [Xiangrui Meng] change NaiveBayesParams to Params
fe23dcb [Xiangrui Meng] remove main from KMeans and add DenseKMeans as an example
67f4448 [Xiangrui Meng] remove main methods from linear regression algorithms and add LinearRegression example
b066bbc [Xiangrui Meng] remove main from ALS and add MovieLensALS example
b040f3b [Xiangrui Meng] change BinaryClassificationParams to Params
577945b [Xiangrui Meng] remove unused imports from NB
3d299bc [Xiangrui Meng] remove main from LR/SVM and add an example app for binary classification
f70878e [Xiangrui Meng] remove main from NaiveBayes and add an example NaiveBayes app
01ec2cd [Xiangrui Meng] Merge branch 'master' into mllib-main
9420692 [Xiangrui Meng] add scopt to examples dependencies