Change working dir away from temp in saveGIF

Hi, this is a follow-up to my last pull request (see https://github.com/yihui/animation/pull/30)

This fix is more complete and it sets the right working directory before calling im.convert. Then, im.convert is able to recognize the ani.options('outdir') and output to the right outdir relative to the working directory instead of the tempdir. This change is in contrast to the last pull request which manually copied files from tempdir to the outdir. This is unnecessary since im.convert recognizes the correct outdir by default, and i did not have to make any changes to code except saveGIF

There are many intermediate commits in this pull request that i used for testing in order to call install_github from devtools. Hopefully this isn't a problem!