Single-user completion certificate generation

* CourseCertificate model created
* Initial cert creation interface through manage.py for one-off cert
  production, s3 upload and (optional) email notification
* UserProfile updates to reflect certificate attachment, move 'Change
  Info' buttons to top of page.
Strip command-line arguments to cert generation
Certificates additional user input error checking
Change certificate to statement of accomplishment

* Change the text of the User Profile page to reflect the fact that we
  don't give out certifications, we give out statements of
  accomplishment.
* Similar updates to manage.py command help.
Certificate Asset downloader for cert deployment

* Downloads assets for statements of accomplishment from the release
  bucket using secure credentials and stores them on localhost for use
  in certificate generation.
install bits for generating certs (#1424)

for @jrbl. Currently only xhtml2pdf.
Minimal UI for new cert creation

* Adds ./manage.py target, create_certificate, which lets you establish
  the database relationship signifying the existence of a particular
  kind of statement of accomplishment.
Cosmetic improvements to user profile page

* Cosmetic improvements to presentation of statements of accomplishment
  on user profile page.
* Introduction of new template utility filter, uscore2space, which
  converts underscores in the input string into spaces.
Documenting certification dependencies
Bugfix to certificate asset downloader

* Target directories weren't being created, so if they didn't already
  exist, the download would fail with an exception. Now we recursively
  create any missing directories in the target hierarchy.
wrong resource in cert recipe (#1424)
Merge pull request #2038 from Stanford-Online/jrbl-1424-certificate-generation-single

Single-user completion certificate generation