add new pipeline methods to get file/image/thumbnail paths

This change allows to pass request, response and spider context to filename
construction methods.
add deprecation warnings for old file_key/image_key/thumb_key methods
better marker name to detect overridden methods
simplify porting to Python 3
switch existing pipeline tests to use new file_path/image_path methods
rename key to path in FilesPipelineTestCase
rename FilesPipelineTestCase image_path var to file_path for clarity
implement DeprecatedFilesPipelineTestCase
implement DeprecatedImagesPipelineTestCase
rename ImagesPipelineTestCase image_path var to file_path for clarity
test for new response and info keyword arguments
fix missing keyword arguments for file_path in media_downloaded()
Merge pull request #490 from max-arnold/master

add new pipeline methods to get file/image/thumbnail paths