Issue #88 - Add ValueRowReader to make it possible to create RDDs of types, for which a TypeConverter exists

[f4bba17de8fbf0ff15d9c4f3229e22af989ef08c] #88 add ValueRowReader to create RDDs of primitives

[e8f33df25b4a4842965797cdc57d5b3201091fee] #88 Made ValueRowReader work with all the types handled by TypeConverter, except for Tuple2

[e11717057091ad915d448f5be820a25527e59a55] #88 Addressed reviewer's comments

 - moved ValidRDDType to a separate file together with implicit methods
 - added example for types disjunction
 - improved compiler comments when implicit RowReaderFactory or ValidRDDType cannot be found.