Allow multiple files containing different tables

Update DbUnitRunner so that multiple IDataSet files are collated into
a CompositeDataSet before operations are applied.

Prior to this commit, when db was in dirty state (possibly after
previous tests run) and we had more than one database setup file then
tables from second and further files were not cleaned up before insert.

Fixes gh-27
Closes gh-56
Merge pull request #56 from mutyonok/master

* pull56:
  Allow multiple files containing different tables