added test for ColumnCombinationBitset compareTo()
implemented compareTo()
Merge branch 'master' of https://github.com/HPI-Information-Systems/Metanome
added testcase for minimal subsets
implemented method to find all minimal subsets
small code refactorings
changed return type from arraylist to TreeSet
using compareTo testhelper for compareTo test
Merge branch 'master' into SubsetGraph
Merge pull request #229 from Jens-Hildebrandt/master

added compareTo() for ColumnCombinationBitset
Merge pull request #234 from Jens-Hildebrandt/SubsetGraph

Extended subset graph
Merge branch 'master' into db-schema-description