Add non-downloading inline_file_view

To be used in documentation for issue #80
Add documentation for serving a file inline.

Responsive to issue #80
Psycho-rebased branch 80-no-attachment-docs on top of master
Refs #80 - Changed wording in 'serve file inline' documentation.
Refs #80 - Improved documentation about 'DownnloadMixin.attachment' attribute.
Refs #80 - Referenced options inherited from DownloadMixin in each view's documentation.
Refs #80 - Added a test around demo's 'object:inline_file' URL.
Refs #80 - Updated changelog.
Improved documentation around 'DownloadMixin.attachment': how to serve files inline. Closes #80.