Initialize Rack::Sendfile with accel mappings

Instead of requiring the X-Accel-Mapping header to be sent by nginx,
this allows separation of concerns between application and server
configuration.
Make spec_sendfile work with Ruby < 1.9
Additional Ruby 1.8 compaitiblity

Mappings are arrays of arrays to preserve order.
Define to_path on File objects in spec_sendfile.
Set TMPDIR to fix Dir.mktmpdir under jRuby
Straw man for rack.hijack*, connection hijacking!
Add straw man for the after-headers hijack
Rename by to store to match better the purpose of that object
Session is already loaded at this point
Implement the ID API correctly
Fix current_session_id docs, id it's retrieved from SessionHash now
Update README.rdoc
Merge pull request #489 from spastorino/abstract_id_tiny_patches

Abstract id tiny patches
Merge pull request #490 from slivu/master

Added Reel Web Server and Espresso Framework to supported software list
Allow subclasses build a different Session Object
Merge pull request #491 from spastorino/allow_build_different_session_object

Allow subclasses build a different Session Object
Enable unix sockets with paths as trusted proxies

 * Supersedes and closes #488
Merge branch 'sendfile_path_mapping' of git://github.com/Casecommons/rack into sendfile

* 'sendfile_path_mapping' of git://github.com/Casecommons/rack:
  Set TMPDIR to fix Dir.mktmpdir under jRuby
  Additional Ruby 1.8 compaitiblity
  Make spec_sendfile work with Ruby < 1.9
  Initialize Rack::Sendfile with accel mappings
Revert "Set TMPDIR to fix Dir.mktmpdir under jRuby"

This reverts commit 3b24fa958ba43baad66c7e866990e052497c89f8.
Minor code review comments closing #451

 * Adds documentation
 * find is preferred over detect
 * Superseded body objects should be closed
Introduce Rack::Mime.match?, references #443