Simplified the way separate header is extracted in multi-file case

in the batch importer, where multiple files are seen as one Readable, and
so the header is always extracted from the first line of the data stream.
Although there's still support for getting the header from a completely
different source.
Moved generic option interpretation into Args
Added the concept of input groups in batch importer

where one group consists of one or more files, logically seen as
concatenated by the batch importer. The first line of an input group
defines the header. An import can have multiple node/relationship groups.

For the command line tool this looks something like:

  ./neo4j-import --into path/to/db
                 --nodes file1;file2;file3;...
		 --nodes file4;file5;...
		 --relationships file6;file7;...
		 ...
Merge pull request #3384 from tinwelint/2.2-bip-input-multiple-files

BatchImport input multiple files