Merge pull request #1 from rstudio/exprdep

Cleaner implementation of expressionDependencies
Clean up output and improve robustness during source package download and installation
Add option to specify local source package locations for packing
Initial implementation of addPackages
More accurate detection of bootstrapping of package
Fix broken lockInfo calls
Implement add(). Pretty sure this is wrong.
Initial support for Bioconducter packages and repos
Version format for Packrat DESCRIPTION file
Add GitHub annotations in DESCRIPTION files for source pkgs
Add \n to end of DESCRIPTION file before appending

write.dcf isn't smart enough to add a newline to the end of a file
that doesn't have one already. This can cause the collate field (or
whatever field comes last) to break.
Only add newline to DESCRIPTION if it's needed
Use locally built package binaries where available to speed install time
Refine status logic when detecting only-in-source packages
Fix error message "argument is of length zero"
Append InstallAgent field to DESCRIPTION files in binary packages we install
Avoid installing already installed packages after initial bootstrap
Initial implementation of package upgrade/downgrade
Refactor install phase to generate list and then play list
Package record manipulating functions