[README] Improve developer instructions

Add a longer tutorial that should cover everything needed to start developing IEs.

Fixes #2676
[README] Add md5 to code example
Merge pull request #2681 from phihag/readme-dev-instructions

[README] Improve developer instructions
[README] Fix VALID_URL in extractor example
[motorsport] Add extractor (Fixes #2688)
release 2014.04.04
[morningstar] Add new extractor (Fixes #2687)
release 2014.04.04.1
[tests] Fix YoutubeDL tests

Since bec1fad, the id, title, and url (also in formats) keys are mandatory. Change the tests to reflect that.
[generic] Remove test case that has become a 404
[dailymotion] Fix playlist+user
[cnet] Fall back to "videos" key
release 2014.04.04.3
[youtube:playlist] Fix playlists for logged-in users (Fixes #2690)
release 2014.04.04.2
[downloader/common] Use `compat_str` with the error in `try_rename` (appeared in #2389)

Otherwise on python 2.x we get `UnicodeDecodeError` because it may contain non ascii characters.
[rts] Add more formats and audio support (Closes #2689)
[UstreamIE] [generic] Added support for Ustream embed URLs (Fixes #2694)
[pornhd] Fix extraction (Closes #2693)
[pornhd] Update test checksum