Fix some paths using relative realpaths() instead of the constants in bootstrap.

This fixes #946 and helps with #894
Get themes dir from config.
Fix issue with $config in paths().
Add caching for dynamic requests

The current code doesn't allow caching for dynamic requests (e.g. requests with pagination, filters, and so on). This behaviour occurs because of the caching key was generated only of the path info. The query string was ignored.

If the cache is invalidated and one browses a moment later to page 2 of a request for an example, or applies an filter to an collection, other visitors of the page would see this users settings for the rest of the cache lifetime.
 Set the frontend pager <ul> class to 'pagination' so it works with Foundation 5
Adds type comparison in Bolt\Config::get()

When evaluating if a value is null in the config it should do a type comparison. I'm having a issue when a set `session_use_storage_handler: false` https://github.com/bolt/bolt/blob/master/app/config/config.yml.dist#L213 and in the https://github.com/bolt/bolt/blob/master/app/src/Bolt/Application.php#L53 is returning `NULL`
Pinterest 'Pin it' button extension
Merge pull request #947 from helderco/bolt-in-vendor

Fix some paths using relative realpaths()
Merge pull request #950 from tschreiner/master

Add caching for dynamic requests
Merge pull request #951 from GawainLynch/master

 Set the frontend pager() <ul> class to 'pagination' so it works with Foundation 5
Merge pull request #952 from andersonamuller/patch-1

Adds type comparison in Bolt\Config::get()
Merge pull request #958 from GawainLynch/master

Add an extension to display Twitter feeds in site templates