Added server and php section to info being tracked

Also:
- Added db_charset to the site variables being monitored
- Moved the server/php specific variables out of the WPSEO specific section to the more generic section as independently of which plugin is using the tracking class, this info is useful and available

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**Rationale behind the newly added tracking variables:**

* **_db_charset and pcre utf-8 support testing_**: helpful info to improve the text analysis meta box section and to solve the bugs people are currently experiencing with this
* **_webserver section_**: frequently needed info when issues are being reported, will be helpful to prioritise issues
* **_webserver - server protocol_**: currently we sometimes hard serve a protocol, we may need to make this flexible wherever this is used. This will help determine whether that is really needed.
* **_php section_**: a number of the white screen reports after the 1.5 release were caused by extensions not being supported. This will help to determine whether we can count on certain extensions being enabled or not and when to stop supporting crappy webhosts which disable essential extensions ;-)

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@petervw @jdevalk:
Not sure how you're catching the info now, but what with the new info being tracked, the catching mechanism may need to be updated too. Also: please take note that I've moved two variables from the wpseo/options section to the new server/php sections.

In that respect, there are now two checks for the curl version and for whether PCRE has utf-8 support. I suggest we keep an eye on those for a little while and if the results are consistent and the same, we remove one of the checks for each and merge earlier retrieved information.