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/1626673/5249308/4c80691a-7982-11e4-931e-aeb67f7dea51.png)


SimpleActivity:
![simple](https://cloud.githubusercontent.com/assets/1626673/5249311/51a9e984-7982-11e4-83f6-fae79dd6dd38.png)

Sharing on Twitter from the SimpleActivity:
![twitter_share](https://cloud.githubusercontent.com/assets/1626673/5249316/6023dd8a-7982-11e4-8e7d-76523e64dd2d.png)


AdvancedActivity:
![advanced](https://cloud.githubusercontent.com/assets/1626673/5249322/6439150c-7982-11e4-91d6-fd8e75fb38ba.png)

Sharing on the default activities from the AdvancedActivity:
![default_share](https://cloud.githubusercontent.com/assets/1626673/5249334/7590f89c-7982-11e4-9c4e-0c41bc639a28.png)


this desc is better used for the jcenter release in pr #10