Fix #2: Can't compile json gem on stock setup

Instead of using the lower-level json gem which requires ruby.h and
possibly more, just require json/pure with no external dependencies. It
may not be as performant as the json gem, but this is hardly measurable
compared to the time it takes to just retrieve the data via the
appFigures API over the net.
Add currency setting under Configuration

AppFigures allows the user to select their preferred currency under the
Account Settings. This is not part of the Sales Report, but the user
can now easily duplicate the setting.
Merge pull request #4 from osteslag/master

Fix #2: lower external dependency requirements. Added currency configuration.