Proposed changes to Configuration checker (LowlevelChecks)

This should be a *mainly* uncontroversial improvement to the way a Bolt app gets bootstrapped and checked. 

The main improvements are that the configurable paths are used so you won't get fatal errors if you move your cache or config directory and also if you install Bolt as a composer package then you can still run the standard bootstrap.

There's a little streamlining of the bootstrap file, in that the Bolt\Configuration\.... class now verifies itself so at its most basic a Bolt app can be bootstrapped using just the following.....
```
$config =  new Bolt\Configuration\Standard(__DIR__);
$config->verify();
$app = new Bolt\Application(array('resources'=> $config  ));
```

Controversial things that could do with  some discussion:
1. To reduce cross-dependencies I've moved the styling for the low level error template inline, this removes some complexity in needing to know web paths to link to stylesheets. The downside may be that styling choices wont automatically cascade to the error templates.
2. I've proposed removing the .htaccess file check. Since this is Apache specific (and modern Apache setups will normally use FallbackResource instead of htaccess), it seems harsh to fatal error the app if you don't have an .htaccess file present, However I'm sure it can be a common problem for users so maybe we can address this check in another way - perhaps a starting point would be to check the server signature to see if they are using Apache? Comments welcome.