Removed NumPy and NLTK from hard build dependencies.

* If the NER example is loaded without manually installing these dependencies,
  the app displays an appropriate error message with a link to documentation.

* The link to documentation for flickr and enron has been updated to point to
  readthedocs instead of localhost (since we don't know what port number we
  should use here, or even if the user has opted to build the documentation).

* This was done to (drastically) speed up the initial build process.  This
  matters because it is annoying to wait several minutes rather than 30 seconds
  for Travis to finish building Tangelo on every commit for continuous
  integration testing.
Fixing serious bug in web content test runner.
Removing unneeded test (NLTK is no longer a hard build dependency).
Merge branch 'no-numpy'