Make sure expr does what we expect under OS X

[hannahwhy]

OS X ships a version of expr that doesn't parse the regexps used by bff.sh. GNU expr does, but that's not present on OS X by default. MacPorts installs a compatible version of expr as gexpr.

bff.sh should make sure that expr does what we expect it to do, and try common alternatives. If no working alternative can be found, bff.sh should halt with an error message.

[jeremydouglass]

When using the current bff.sh script on OS X without changing expr to MacPorts gexpr, you get the following behavior:

1. output log is full of "usage: dirname path" errors, apparently from using dirname with an empty variable in the image saving loop.
2. there are no files in each ~/photos directory. instead, a 0k file named ".jpg" is touched each time a new image should be downloaded.

[hannahwhy]

Friendster's dead, and so is this project. Closing.