Update packages from third-party reposes (e.g. R-Forge) iff not available on CRAN

[HenrikBengtsson]

Create an update mechanism that:

1. Updates from CRAN and Bioconductor.
2. Checks for available updates on R-Forge, rforge and Omegahat. Keep packages that are neither on CRAN nor on Bioconductor. Update the remaining ones.