Update dev setup instructions in README.
Update dev setup instructions in README.

- Add instructions for registering app with OSM to get consumer and
secret keys.

- Fix the environment variable names (which now start with MR_) in
Mac and Linux sections.

- Document the MR_OSM_SERVER environment variable for those who
wish to connect to a development OSM server.

- Include the commands for installing postgres and postgis via
homebrew in Mac section, just for completeness.

maproulette/maproulette2,mvexel/maproulette2,maproulette/maproulette2,maproulette/maproulette2,Crashfreak/maproulette2,mgcuthbert/maproulette2,mgcuthbert/maproulette2,Crashfreak/maproulette2,mvexel/maproulette2