Update PhpAmqpLibMessagePublisher to deal with "headers" key

Complete test

Correct expected format
PeclMessagePublisher must deal with application_headers
Update PhpAmqpLibMessageProvider to retrive headers
Add .gitattributes file

@See
http://www.reddit.com/r/PHP/comments/2jzp6k/i_dont_need_your_tests_in_my_production/
for more informations.
Complete RetryProcessor README
Merge pull request #72 from odolbeau/improve-retry-readme

Complete RetryProcessor README
Merge pull request #71 from odolbeau/add-gitattributes

Add .gitattributes file
Merge pull request #68 from odolbeau/fix-retry-lib

Can now use RetryProcessor with PhpAmqpLib