DON'T MERGE: Make variable naming uniform in R Programming

We've received feedback from several people on the forums who were getting stuck on e.g. `myVector <- ...` because they couldn't tell the `V` was supposed to be capitalized (poor fonts, small monitors, etc). Others have requested we make variable naming uniform, since some lessons use camel caps and others use underscores. 

These changes have not been tested and it's possible I missed something.