Added formatter for volume.

Added a formatter for volumes and updated the exchange detail screen to
use it.
Replaced malloc calls with calloc.

It turns out that the Pebble SDK, even though it’s not in the
documentation, supports calloc. That being the case I replaced the
malloc calls with calloc (even when it didn’t necessarily make senes
because I’m a big fan of known states even if it does cost a little
time in zeroing out memory).
Restored the ability to refresh an exchange price.

The ability to perform a long hold on the Pebble’s select button to
refresh pricing information has been restored. Unlike the prevoius
version this refresh only occurs for the selected exchange. I may
create an option to do a refresh all at some point in the future.