Allow users to extend the SSTemplateParser by defining open & closed blocks

Currently the only way the extend SSTemplateParser is to define a class
extension of it and then tell the injector component to use your new
custom class. This new change allows a user to define new "open blocks"
and "closed blocks" for SSTemplateParser to use without needing to
recompile the real SSTemplateParser class.

The following example shows how the functionality can be used
to add a new <% minify %>…<% end_minify %> syntax to the template parser

In a config.yml file, define the new minify closed block to call the
static function "Minifier::minify"

```
Injector:
	SSTemplateParser:
		properties:
			closedBlocks:
				minify: "Minifier::minify"
```

Define a new class with the minify static method that returns the new
template code when regenerating templates:

```
class Minifier {
    public static function minify(&$res) {
        return <<<PHP
{$res['Template']['php']}
\$val = zz\Html\HTMLMinify::minify(\$val, array('optimizationLevel' => zz\Html\HTMLMinify::OPTIMIZATION_ADVANCED));
PHP;
    }
}
```
Update partial-caching doc to include note on using .Max and .Count together.
Update partial-caching.md
Merge pull request #2656 from madmatt/patch-2

Update partial-caching doc to include note on using .Max and .Count together
Added 3.1.2 changelog
Merge branch 'tmp-3.1.2' into 3.1
Merge pull request #2596 from camspiers/sstemplateparser-extendabilty

Allow users to extend the SSTemplateParser by defining open & closed blocks
Add X-Backurl header to CMS ajax requests
Fixing coding conventions in Behat files
Merge pull request #2657 from kinglozzer/cms-xhr-referer

NEW: Add X-Backurl header to CMS AJAX requests
Fix installer isIIS() to include IIS versions greater than 7.
Merge pull request #2672 from tiaitsch85/tiaitsch85-patch-1
Merge remote-tracking branch 'upstream/3.1' into 3.1