Added LibreJS tags to vendor/assets/javascripts scripts.

Unfortunately, I was unable to find the source code for these files:

vendor/assets/javascripts/mbp-respond.min.js
vendor/assets/javascripts/mbp-modernizr.custom.js

In addition, I was unable to find license information for these
scripts:

vendor/assets/javascripts/handlebars-1.0.0.beta.6.js
vendor/assets/javascripts/parse_url.js
vendor/assets/javascripts/keycodes.js
vendor/assets/javascripts/mbp-helper.js
vendor/assets/javascripts/markdown/Markdown.Converter.js
vendor/assets/javascripts/markdown/Markdown.Sanitizer.js

This needs further investigation.