Add history file that is generated by R.app for Mac OS

In section 2.4.10 of the [R for Mac OS X FAQ](http://cran.r-project.org/bin/macosx/RMacOSX-FAQ.html), it is noted that `R.app` uses an `.Rapp.history` file for saving the history of an R session rather than an `.Rhistory` file.