Add Matrix.readTsv object method

The [Matrix API wiki page](https://github.com/twitter/scalding/wiki/Matrix-API-Reference#wiki-readTSV) promises a ```readTSV``` object method, but none yet exists. I think this is a correct implementation, but with the name changed to ```readTsv``` to be consistent with the casing of ```com.twitter.scalding.Tsv``` method. 

I think it might be better to be able to write code like this (perhaps with required typing):

```scala
val m = Matrix.read(Tsv("input"))
// parallel to:
m.write(Tsv("output"))
```
but I don't know enough to write such a method.