Configurable null values

[petro-rudenko]

There's datasets where each column has it's own marker for missing values. spark-csv assumes only empty string for missing values. To avoid additional data transformation and saving on user's side would be great to specify a set of null markers and replace them to empty string on a library side.

[spark-water]

I ran into the same problem. Here is how I fixed it. Hopefully it would help others

1. remove .ivy2/cashe rm -rf ~/.ivy2/cache

2. redo git clone

3. now go into project root and run build/sbt assembly

you should have a build success (at least for me). Good luck!