Fixed building for iOS6.x.

This would build for iOS7 but not for iOS6.
Fixed a bug in the automatic labeling method that could cause an application hang with very small axis ranges. Fixed issue #52.
Changed some view coordinate calculations to use NSDecimal throughout to reduce rounding errors that occurred with very small plot ranges.
Merge branch 'fixes-to-push' of https://github.com/bignerdranch/core-plot into bignerdranch-fixes-to-push
Added back the run-time check when using the iOS 7 SDK since the app could be running on an earlier system version that doesn't have the new method.