Update the README

Update the readme to refer to 9.1+2.0 by default.
Information for older versions remains as an alternative.
Add example osm2pgsql usage

The example osm2pgsql usage is for a full planet and based on reasonable defaults for rendering osm-carto.

The example will periodically require updating to increase the RAM cache and quoted flat nodes size.

Also, be clearer about supporting PBF.
Merge pull request #40 from pnorman/example_docs

Update the README
Merge pull request #42 from pnorman/example_help

Add example osm2pgsql usage
Explain why fork() failed

In my experience, it's often because the cache size was set too high and fork()
can't allocate sufficient memory for the child. However, if nothing to this
effect is output, it's not clear what can be done.
Message cleanup
Merge pull request #47 from mojodna/fork-error-message

Explain why fork() failed