Added version limits for all deps
Tidied up the interface page a bit

Added link for camera settings, moved Download and Send functions to the
bottom of the page and made their appearance conditional upon images
being available.
Send function now implemented

Added ability to send camera images as e-mail attachments. Needs testing
with SMTP and a sendmail implementation
Added options for video debugging

Now defaults to raspivid/still with option to use GStreamer for non-RPi
debugging. Also added options to specify raspivid/still binaries
manually.
Moved all camera handling to globals

Yup, it's ugly but there's a single camera on the RPi; no matter how
many instances of PicroscopyCamera get instantiated they're all sharing
the same camera so the state needs to be global