readme: fix NextFlowItems

project: folders refactoring

This commit moves the RxFlow project in the root folder to
make Carthage work.

project: bump CocoaPods version

project: iOS 9.0 compatibilty + logo replacement

This commit:
- sets the deployement target to iOS 9.0
- modify the texts and logo of the "About" page
- embed RxFlow framework into RxFlowDemo

fix: rename DemoWeft to DemoStep

project: adapt CocoaPods spec