portable sed commands

[tedivm]

The original sed variant detection worked by checking the operating system type- if the OS is darwin (ie, OSX) it uses the BSD sed, otherwise the GNU sed.

This change instead uses "duck typing" to identify the variant. It runs sed --version to detect whether the linux or bsd version is running. On the OSX/BSD version the sed --version command fails, but on the gnu version it succeeds.

This allows the script to work in two cases where it otherwise fails-

1. For people (like me) who install the updated gnu versions of everything on their OSX machines.
2. On other BSD style systems.

[ekalinin]

Thanks!