Logging functions: level parameter is not optional

This parameter cannot be optional, as the following parameter is mandatory.

Also completed the corresponding documentation.
Fix a docblock in Loader class
Do not trigger a possible custom autoloader, as it is irrelevant here

These were the last two calls of class_exists() without the $autoloader = FALSE argument.
Adds a few new lines to note directive to fix a build error in the docs.
Removes an obstacle to building the docs.
Merge pull request #2440 from vlakoff/develop

Logging functions: level parameter is not optional
Merge pull request #2441 from vlakoff/develop-2

Fix a docblock in Loader class
Merge pull request #2442 from vlakoff/develop-3

Do not trigger a possible custom autoloader, as it is irrelevant here
Add a requirement in tests installation instructions

PHPUnit requires Yaml package from pear.symfony.com channel.
Fix tests execution

See #2442
Try a different method, the previous one wasn't working in Travis
Merge pull request #2462 from vlakoff/develop-4

Fix tests execution
Merge pull request #2460 from vlakoff/develop-3

Add a requirement in tests installation instructions
Merge pull request #2 from EllisLab/develop

Develop