Improve da(1) and devassistant(1) man pages.

Try to follow more conventional man page style e.g. ENVIRONMENT instead
of ENVIRONMENT VARIABLES, "colon-separated list of directories" instead
of "list of paths delimited by colon". Also, these commands are python
programs so replace references to "binaries". Move note in
devassistant(1) from NAME to DESCRIPTION.
Merge pull request #213 from jwakely/man-pages

Improve da(1) and devassistant(1) man pages.