adding missing install option in the text

The `pip`s didn't have `install` option in them,,,
I added them.
Added test for get_page
Add test for grab_hidden_video_url
Update downloaders.py

Fix Issues #283 .srt and .txt files problem
Merge pull request #287 from zlbruce/patch-1

Fix #283, where .srt and .txt files were being written in gzipped format.

The problem here is that we were using the raw response of
`urllib3.response.HTTPResponse` via the `requests` library,
which preserved the transfer encoding of the text files.

Thanks @zlbruce for the fix.
Merge pull request #281 from MHM5000/patch-1

README: Add missing `install` subcommand for the `pip` commands.
Merge pull request #284 from vladistan/more_coverage

Add more tests and increase code coverage.

Thanks @vladistan for the code.
coursera: Update my copyright years.
cosmetics: Fix some PEP-8 issues and some stylistic changes.
Remove unused imports.
Reorder imports to be compliant with PEP-8.
requirements: Update minimum version of requests. Closes #291.