Generate @since data also for parameters.

Only add the @since annotation for params that were introduced after the
method itself was added.  When doing so, only consider the positions of
parameters, as they might be renamed between versions.

When method has explicit @since tag, then no param since tags are added.

Also implement #respond_to? on NullObject.
Move @new tooltip creation out of Process::Versions.

The tooltip creation is really not the task for the Versions processor.
Moving it back to Tag::New were it was initially.
Simplify Process::Versions importer param default.

Just a shorter way to express the same thing.
Turn versions hash into instance var.

Avoiding passing it to almost every method.

We can now also implement an is_new? method that initializes the
lookup hash by itself when needed.
Split up the big method in Process::Versions.
Better organization of methods in Process::Versions.
Let Tag::Since do the rendering of @param versions.

So the logic for rendering the HTML is contained in one spot.
Display @new star next to new parameters.
List methods with new parameters on front page.
Merge branch 'since-params'