Priority share - add readme and demo project

This PR adds a README for the Priority Share library and a demo project

The demo project contains 3 Activities:

 - A main activity that shows 2 buttons:
  - The first one opens the simple demo activity, which allows you to type the text to share and that's about it
  - The second one opens the advanced demo activity, which allows you to customise the share action with all the available options that the lib offers.

Screenshot time!

MainActivity:
![main](https://cloud.githubusercontent.com/assets/1626673/5248193/51ee164e-7979-11e4-9d1c-30a3068a499e.png)

SimpleActivity:
![simple](https://cloud.githubusercontent.com/assets/1626673/5248196/5acec2cc-7979-11e4-8487-c0c72c32bef0.png)

AdvancedActivity:
![advanced](https://cloud.githubusercontent.com/assets/1626673/5248198/608e1c80-7979-11e4-84b1-925ee7043701.png)

but the benefits of priority share are so that you can select where to share as a priority, so I would add to this example (not sure about syntax) `.setApps(TWITTER_CLIENTS);`