Remove wrong explanatory comment
Add test cases for S3 dispatch
Add extensive tests for `is_S3_user_generic`
Add skeleton code for S3 registration

Function `is_S3_user_generic` test whether a given function is a
user-supplied generic.
Implement S3 method registration for user generics
Fix error in test
Add test for re-registration of S3 methods
Check against double-registration of methods
Clarify comment
Fix test cases

The `seq` test case didn’t work since `seq` (and not just `seq.int`) is
already a generic, and does dispatching accordingly. The `se.contrast`
example works since `se` is not (at the moment) a generic in R base.
Simplify `lsf` function
Remove unnecessary step from S3 handling
Document S3 support
Fix roxygen syntax
Merge branch 'S3' into develop
Update version field and change log