Add Nextflow Frontier announcement banner - #502

Changed pages include:

• index.html - view
• 404.html - view
• about-us.html - view
• ambassadors.html - view
• basic-pipeline.html - view
• blast-pipeline.html - view
• examples.html - view
• gsoc2016.html - view
• machine-learning-pipeline.html - view
• mixing-scripting-languages.html - view

There are more changes not included in the list