Clean up package installation process, and provide sessionInfo in README

The package installation process now does not install a package if it is already installed. The list of packages to install has been tested using a completely clean installation of R 3.0.3. The packages listed in sessionInfo() should be all the packages needed for the code to work. There is also a workaround for a bug in the package httr; httr should mention jsonlite and httpuv as dependencies rather than suggestions.