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
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 * Superseded body objects should be closed