[gotporn] Add new extractor

[drguildo]

Before submitting a pull request make sure you have:

• [x ] At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Added support for GotPorn.

$ python test\test_download.py TestDownload.test_GotPorn
[GotPorn] 5661549: Downloading webpage
[info] Writing video description metadata as JSON to: 5661549.info.json
[debug] Invoking downloader on 'http://www.gotporn.com/gvf/q7t29q6cq4hy5bqw06tflwb60vh8qjd7ks12psqwnw1zvgt1dwbzsy1l8w1sy0r7AkglfhvAlvldAqz7hAzbmsjbr24kxj21hsp40kyngr0l9xc492x7k5kvc1vgwrw6p3rl87hybn22mwAzArscwkmy5f7txAjqfsl0pnt8w34n0vh059ky3mA8kpyvdAmg6mfjrpxw344n9851k4sv725zbhy4A3kc246bx8641b9cc18flkxttxvb8mzddjxhr1pmclctgxczrl3w9fq2bsp8jjhtAlvzsr8bxf9sg8bbzpt144cq'
[download] Destination: 5661549.mp4
[download] 100% of 10.00KiB in 00:00
.
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Ran 1 test in 5.857s

OK

[dstftw]

It's well covered by generic extractor.