Fixing setting speed limit of torrent in WEBUI

Fixing setting speed limit of torrent in WEBUI when global speed limit
is not set.
Update priority icons

Update priority icons
Change indentation to 4 spaces. Issue #2192.
WebUI: Assign a version to the Web API and allow to get it

New GET methods:
 * version/api: current API version
 * version/api_min: min API version supported

The API version is an integer and it's returned as plain text.

Whenever the API is changed, the api version is increased.

Whenever the API is changed in a way that is not backward compatible,
api_min is increased.
WebUI: Allow to retrieve qBittorrent version

New GET method:
 * version/qbittorrent: qBittorrent version

Closes #1872.
Show speed in title
Change of number of decimal places to 1

Change of number of decimal places to 1 in speed and size values.
Merge pull request #2125 from buinsky/master

WEBUI: Some fixes
Merge pull request #2153 from pmzqla/webui-versioninfo

WebUI: Allow to retrieve web API version and qBT version
Follow project coding style. Issue #2192.
Stop using internal libtorrent API. Closes #2202.
Use parse_magnet_uri() where possible.
Remove useless includes.
Revert "Temporarily disable Travis CI tests against libtorrent 1.0.x until #1878 is resolved."

This reverts commit 7029f4dcd2f123014b93718ee8da68ce8e1558e7.