cleaned up `updatePackages` examples
add unit test for `addOldPackage` (#25)
renamed functions to add packages (#21)

`addPackage` ==> `addPackageListing`
`addPackageGithub` ==> `addPackageListingGithub`
`add.packages.miniCRAN` ==> `addPackage`
fix error: `addPackage` & `addOldPackage`
`addOldPackage` unit tests (#25)
`updatePackages` unit tests (#29)

*need tests for binaries*
suppress warnings in unit test (#29)
add unit test for `checkVersions` (#28)

*needs binary tests*
clean up examples and rebuild docs
rebuild docs for renamed functions

with commit 7081c6f:

`addPackageGithub` ==> `addPackageListingGithub`
clean up `addPackage`

- improved messages when removing old versions
- only create dir structure if not already present
fixed test_all errors (#25, #28, #29)

added missing repos as needed
add unit test for `addPackage` (#29)