enable newrelic and set config file via environment variable + documentation

Fixed some stuff that wasn't correct in the README, mainly that the collective.newrelic egg needs to be in auto-checkout to have it generate newrelic-admin script in the bin/ directory. This happens regardless of having the egg in development mode. Removed the mentioning source directories where newrelic looks for the .ini file, since this was misleading and/or not entirely correct, see issue 12. 

Also patched __init__.py in patches directory as to enable setting the newrelic.ini file via the NEW_RELIC_CONFIG_FILE environment variable (see issue https://github.com/collective/collective.newrelic/issues/12 , last comment). 

Updated the README file with version pins that work for me, and documented how to configure the product using environment variables to enable newrelic itself as well as setting newrelic.ini. 

The only thing I am not sure about is the version pin of collective.newrelic itself in the README file: I let it stay 1.0.8 since that is the base version for this pull request, but of course the change in the code can only be effective on a next release, so I put a comment above it saying the version number should be bumped to 1.0.9 on the next release for the documentation te be accurate. 

That is, if my pull request gets accepted :)