Remove duplication when generating list of entries to process

Posts and Drafts share the same logic to get the list of files to
process into objects when generating the site. Factor this logic into
its own method and use it when reading posts and reading drafts.
Remove duplication when aggregating post information
Added mime.types file from Apache SVN repo.
Jekyll::Commands::Serve reads in mime.types file.
Merge pull request #847 from mojombo/mime_file

Load in Apache MIME Types
Update history to reflect merge of #847
Merge pull request #838 from mattr-/dedupe-entry-generation

Remove code duplication during site generation