Add note to readmes about storing refresh tokens

Fixes GH-61.
Add CocoaPods badges to READMEs
Update CocoaPods badges to point to HTTPS endpoints
Add CocoaPods upstream podspec and Rakefile to project
Implement rake spec to run unit tests on all platforms
Add macro to BoxLog.h to disable DEBUG build logging

Add documentation about DEBUG assertions and logging to BoxSDK.h
and BoxCocoaSDK.h.

Fixes GH-64.
Rakefile doesn't like single quotes in podspec
Update Rakefile to generate tags prefixed with 'v'.
Release v1.2.1
Fix podspec definition to include all headers and create public headers

The previous podspecs did not include BoxSDK.{h,m}.
Release v1.2.2