Revise timeout settings

[jojoob]

The changes of this pull request introduce the two new settings 'tool_opencast/apitimeout' and 'tool_opencast/apiconnecttimeout'. Both to be set in milliseconds. 'tool_opencast/apitimeout' controls 'CURLOPT_TIMEOUT_MS' and 'tool_opencast/apiconnecttimeout' 'CURLOPT_CONNECTTIMEOUT_MS'.

The setting 'tool_opencast/connecttimeout' was dropped because it never had an effect (see #41) and the unit were seconds.

I manually tested the patch with netem/tc

During my work on this patch I discovered the following todos (which should be handled in separate issues/PRs):

• improve curl error handling
  • no user friendly error message is shown when the API call fails
  • block_opencast: every API call is executed independently, i.g. three calls are made to load the block_opencast video overview page resulting in a page load time of timeout * 3 when there is a problem.
• revise the $customconfigs parameter handling (currently $customconfigs is an all-or-noting decision, not possible to only set 'apitimeout' for example)

[TamaraGunkel]

Hey,
thanks for your work!
And you've definitely a point with your todos. We should refactor the code in the future. I'll create an issue for it.