caching: Sample proxy_cache_key construction.

Adding a nginx conf snippet for constructing a proxy_cache_key based on the UserAgent and expected optimizations for that UserAgent class, bypassing the cache for all other cases. Sample config for downstream-cache proxy_cache_key construction now takes care of 2 classes of UserAgents (expected to get most of the traffic for a site) and also bypasses cache for all other desktop UAs, all mobile UAs and all tablet UAs.

(Fixing the startup phase of the nginx server to put out the original error message in case of failure.)
Merge pull request #409 from pagespeed/anupama-update-trunk-tracking-html-caching

caching: Sample proxy_cache_key construction.