rename functions using lowerCamelCase

`old.packages.miniCRAN` ==> `oldPackages`
`update.packages.miniCRAN` ==> `updatePackages`
`check.package.versions` ==> `checkVersions`

**not done:** `add.packages.miniCRAN` (wait for decision on how to
merge with `addPackage`)

RStudio removed trailing whitespace
rename `avlPkgs` argument to `availPkgs`
use separate Rd files and documentation cleanup
reordered `pkg` and `path` args to match `makeRepo`
split long lines
export `repoPrefix` and `twodigitRversion`

added documentation for these two functions
fix paths to example files

use forward slashes (instead of backslashes) for OS-independent package
building
updated and improved examples; fixed typos

* moved examples to `inst/examples`
* made examples platform independent
* added code to make each example stand-alone
* minor clean up (e.g., whitespace)
* fixed typos in `addPackages.R` documentation
updated imports in `twodigitRversion`

added `@importFrom methods is` to `twodigitRversion`
rebuilt .Rd files and NAMESPACE
whitespace clean up
clean up `checkVersions` and `add.packages.miniCRAN`

* fixed documentation to refer to correct return value of
`checkVersions`
* ensure `Rversion` is 2-digit in `add.packages.miniCRAN`
undo export of `repoPrefix` and `twodigitRversion`

rejiggered their use so that they are only used internally
added function to add old package versions

* added `addOldPackage` function to get older source version from CRAN
and add the miniCRAN repo
* updated examples
rebuilt .Rd files and `NAMESPACE`