Corrected i18n: "Search Engine Optimization"

Corrected the i18n for Magento Core and Install to change *"Search Engines Optimization"* to *"Search Engine Optimization"* by removing the *"s"* from *"Eninge"* in all i18n CSV files and relevant configurations as it is most commonly spelt (and spoken) this way - A quick Google search for *"SEO"* can confirm this.

Tested against Magento 2.0.0.0-Dev64 (271615abec) on Ubuntu - Changes are reflected as expected without errors or warnings (Developer Mode Enabled)