[SPARK-1752][MLLIB] Standardize text format for vectors and labeled points

We should standardize the text format used to represent vectors and labeled points. The proposed formats are the following:

1. dense vector: `[v0,v1,..]`
2. sparse vector: `(size,[i0,i1],[v0,v1])`
3. labeled point: `(label,vector)`

where "(..)" indicates a tuple and "[...]" indicate an array. Those are compatible with Python's syntax and can be easily parsed using `eval`. This PR doesn't contain changes to pyspark. I need to figure out how to change dense array's `__str__`.

`MLUtils#saveLabeledData` and `MLUtils#loadLabeledData` are deprecated. Users should use `RDD#saveAsTextFile` and `MLUtils#loadLabeledPoints` instead.