Multi file datasets

I'm enjoying using CassandraUnit but have found that working with just one file to contain a DataSet is a bit painful - especially if I end up having to copy-paste sections of my data into other files. 

I've written a new `DataSet` implementation called `MultiSourceDataSet` that has two static constructor methods:
```
    public static MultiSourceDataSet fromClassPath(String... classpathFileNames)
    public static MultiSourceDataSet fromFiles(String... fileNames)
```

This allows a full `DataSet` to be created by "merging together" the column families found in each file. The only rules are:
  - Each column family must belong to one keyspace
  - Column families cannot be defined multiple times

I've written tests for all the combinations of success/failure I can think of - hope you like it!