GeneralTestBF: denominator argument and names<- replacement method

This pull request adds two things (and some things to the .gitignore file that can be considered collateral damage):
- A denominator argument to generalTestBF against which all tested models are compared to (however, the old behavior remains intact).
- A names replacement methods to objects of class BfBayesFactor. This also promoted the names method to an S4 method (was S3).