Improve meta tags for splash pages

This commit improves the meta tags on GoCardless splash pages.
* Optimise the title tag (e.g. "API - GoCardless" now becomes "GoCardless API: Integrate Direct Debit into your website & app - GoCardless"). 
* Remove the default meta description ("GoCardless is the UK's #1 for Direct Debit, serving more companies than any other provider. Perfect for recurring billing and B2B invoicing.") - [better to show no meta description rather than duplicates](http://searchengineland.com/googles-matt-cutts-dont-duplicate-your-meta-descriptions-177706).
* Added a custom meta description on relevant splash pages (e.g. on the API page: "With the GoCardless REST API, you can integrate Direct Debit payments into your website or app. Official libraries for Ruby, PHP, Java & more.")
* Added a noindex,follow on five splash pages: /demo, /request-a-quote, /request-dd-guide, /request-sage-demo and /request-sepa-updates. These have very little content and we don't want them competing with our main landing pages (e.g. /watch-a-demo, /sage and /europe).

Before and after can be found in the Marketing Dropbox (Splash Page Meta Tags.xlsx in SEO Folder)

![html title and desc](https://cloud.githubusercontent.com/assets/6426688/3845654/ed0bfeb4-1e54-11e4-8e8b-ecc32ba9e342.png)

Useful links for reference:
* http://moz.com/learn/seo/title-tag (title: 50-60 characters)
* http://moz.com/learn/seo/meta-description (description: up to 155 characters)
* http://googlewebmastercentral.blogspot.co.uk/2007/03/using-robots-meta-tag.html (noindex, follow)


Should change it to be description and robots specifically:
```
{% block description %}Desc{% endblock %}
{% block robots %}noindex,follow{% endblock %}
```

For robots you can conditionally set the meta tag if it has a value.
Better to provide a simple way to do this stuff leaving less room for error.