Merge features from dev to master

See merge request !14

Merge features from dev to master

See merge request !13

Checks if tqdm module is installed and if so, defaults to it for
displaying progress

Otherwise uses old, self-made progressbar

In anticipation of addtional progressbar logic

The function isn't defined at that point yet which would cause a
separate exception if the requests package isn't installed.

The webbrowser package is in the python standard library so it should
always exist

Merge features from dev to master

Closes #5

See merge request !12

Add "FullHD" -> 1080p and "SD" -> 480p keywords

Closes #5

e.g. "1080p" or "highest"/"lowest"

Part of #5

If the resolution wasn't a keyword, we would get a reference error due
to never seeing the variable name before

...to own function

Part of #5

Merge features from dev to master

See merge request !11

Authentication later on will break if the referer is
`https://www.video.ethz.ch/...` instead of `https://.video.ethz.ch/...`

Lectures with no video uploaded yet will give a 404 HTML site even on
the `*.series-metadata.json` path. Naturally this will cause the JSON
parsing to fail. We still might want to download other lectures though,
so we just skip this particular one.

Merge features from dev to master

See merge request !10

...between episodes

The return value of `get_video_src_link_for_resolution()` would override
user choice for the next episode with the actual value of the current
one.

Accidentally copy and pasted the test names without changing them

Two videos were previously always downloaded in `low` resolution but
there actually existed 4 options which meant that previously `low` was
not actually the lowest quality

Instead of vague `high`, `medium`, `low` which didn't always line up
with what exists, use exact resolution instead.
If exact resolution doesn't exist, use the next closest one.
Filenames now include the value of the actual resolution.
Keywords `highest` and `lowest` allow downloading the highest and lowest
quality respectively
Default resolution is HD as it should provide the optimal trade off
between high enough for small text in video to be readable while not
straining users' devices and connections or ETH's servers.