Include installation documentation in the repository

Based on discussions at EWG, it was decided to move the setup documentation into the repository. This PR features:

* Reviewed, tested and reworked installation notes
* Splitting contribution-related materiel into CONTRIBUTING.md
* Post-installation optional configuration steps in a separate (cross-platform) CONFIGURATION.md file
* Removal of some crufty bits of the old wiki docs

It doesn't include:

* Platform-specific installation notes. MacOSX and Fedora are anticipated
* Clarification by renaming the databases (to osm-development etc) - for a future PR, perhaps

If this PR is merged, then I will remove the stale documentation from the wiki and leave pointers to these in-repo documents.