Make the check for whether the 'edit files' admin page should be shown consistent

Check was being done differently in three different places, now changed to consistent function call.
Remove `function_exists( 'is_multisite' )` check

As min requirement for the plugin is nowadays well above WP 3.0, the check for `function_exists` on `is_multisite` is no longer needed, removed these elsewhere.
Changed the check for file editing admin screen to use WP native `current_user_can`
Updated changelog
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive

Conflicts:
	readme.txt
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive
Refactor replace vars: move away from singleton pattern

-> Changed a few properties and methods to static to make them available to all instances while only having to set them up once (efficiency). Applies to the registration of new replacement variables and the help text methods
-> Changed class calls to be in line with this change

Also: fixed issue where post title replacement was still playing up, this will probably fix the issues people using the Optima Express plugin were still having
Refactor replace vars: removed superfluous array cast as this is done on the receiving end anyways
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Replace-vars-no-more-singleton
Added filter `wpseo_replacements_filter_sep`
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive

Conflicts:
	inc/wpseo-non-ajax-functions.php
	readme.txt
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Replace-vars-no-more-singleton
Add filter documentation for new 'wpseo_replacements_filter_sep' filter
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1197-file-edit-directive

Conflicts:
	readme.txt
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Replace-vars-no-more-singleton

Conflicts:
	readme.txt
Merge pull request #1250 from jrfnl/Replace-vars-no-more-singleton

Replace vars - no more singleton
Merge pull request #1209 from jrfnl/Issue-1197-file-edit-directive

Issue 1197 - consistent file edit permission check
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1158-tax-index-dropdown

Conflicts:
	readme.txt