Create update-packages.R

add two new functions to check for old packages and update the miniCRAN versions:
* `old.packages.miniCRAN()` follows code/syntax of `utils::old.packages()`
* `update.packages.miniCRAN()` follows code/syntax of `utils::update.packages()`
* the update function calls `add.packages.miniCRAN()` in `add-packages.R`
added additional example
extend 'twodigitRversion' to handle more types

allow passing of `R_system_version` and `character` class objects to
`twodigitRversion`
add .gitignore and .Rbuildignore
added functions to add pkgs to existing miniCRAN

* added new functions `addPackages.R`:
-- `add.packages.miniCRAN` adds pkgs to existing miniCRAN repo
-- `check.package.versions` reports full filepaths of package files
currently in the miniCRAN repo directory
* added call to `twodigitRversion` into `repoPrefix` where it's needed
* removed call to `twodigitRversion` from `makeRepo`
* added checks to make sure user specified 'path` exists
removed ref to "nrcran"
added @import tools to `update-packages.R`
fix pkg comparison error `add.packages.miniCRAN`
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`