Add message about Google Chrome being 32-bit

I've added a message in red to the documentation's index explaining this
problem (for more details, see below).

After an e-mail exchange with Chromium engineers we've figured that
there are some cases where the amount of samples in a study requires
a 64-bit version of Google Chrome which is only available when running
under Linux as that's the only officially supported platform for 64-bit
builds of Google Chrome. See the following link:
    https://code.google.com/p/chromium/issues/detail?id=312958

There are open tickets in their issue tracker trying to target the
release of a 64-bit version for OS X, however this has not been
addressed yet:
    https://code.google.com/p/chromium/issues/detail?id=18323

It's worth adding that in previous versions of Google Chrome this didn't
matter and visualizations would work without a problem, hence the
suggestion now is to open the bigger studies using Safari or Firefox
(which we've tested and work just fine).
Change message to be clearer and link to issues
Merge pull request #214 from ElDeveloper/chrome_notification

Add message about Google Chrome being 32-bit
Merge branch 'master' of git://github.com/qiime/emperor into issue_150
Add combo boxes for the animation categories

The user can now select the category by which the trajectories will be
sorted and the category by which each category will be created.