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`
Fix: taxonomy meta field description was only shown for text fields
Fix issue #1158 - Show taxonomy index meta field regardless of blog-wide setting
esc_html the tax met field description on function call, not within function to allow passing some html
Updated changelog
Updated changelog
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-1158-tax-index-dropdown
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-1158-tax-index-dropdown

Conflicts:
	readme.txt
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-1158-tax-index-dropdown
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 Issue-1158-tax-index-dropdown
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Replace-vars-no-more-singleton
Added filter `wpseo_replacements_filter_sep`