Added the HttpFoundation-based fixture for the error 500 page

Ref https://github.com/Behat/Mink/pull/500
Merge pull request #52 from stof/test_error500

Added the HttpFoundation-based fixture for the error 500 page
Fixed the conversion of header names to server keys to support any header
Refactored the handling of headers

The headers are kept in the driver and passed to the BrowserKit client
when sending the request to avoid altering the state of the client itself.
This allows resetting them properly when resetting the driver.
Improved the Travis build matrix

Tests again the different PHP versions are now resolving the composer
dependencies unmodified. Tests again other Symfony versions are runing
only for PHP 5.5 to limit the number of jobs in the matrix.
Testing against PHP 5.6 and against Symfony 2.5@dev has been added.
Improved the conversion of header names to SERVER keys

- AUTHORIZATION is now sent as a normal header rather than being converted
  to PHP_AUTH_DIGEST. This was a weird conversion done for Symfony years
  ago but the AUTHORIZATION header support has been fixed in
  HttpFoundation since then.
- The CONTENT_* headers are not prefixed with HTTP_ to match the behavior
  of PHP.
Fixed the submission of empty textareas

Closes Behat/MinkGoutteDriver#10
Merge pull request #55 from stof/simplify_travis

Improved the Travis build matrix
Merge pull request #53 from stof/fix_request_header

Fix handling of request header
Merge pull request #56 from stof/fix_empty_textarea

Fixed the submission of empty textareas
Changed exception to use a DriverException when the XPath does not match

The NotFoundElementException is meant to be used by higher level APIs (the
Element API), not by drivers themselves.
This makes the driver consistent as some checks were using DriverException
and some others were using NotFoundElementException.
Added some safeguards to throw an exception on invalid usage

A DriverException when invalid form actions are attempted on an element
is better than a fatal error.