Update asset_pipeline.md

The nginx examples for far-future headers and gzipped static resources were out of sync. 

The far-future headers section used 'expires 1y' instead of 'expires max'. Also added a link to the expires nginx and apache modules rather than provide large amounts of duplicated explanation here for various options.

The gzip block had an unnecessary and potentially confusing 'root' directive not present in the far-future headers section.