Merged pull requests

Some Pull Requests have been merged to provide new features. The most relevant are:

• Implemented support for multiple Pin image URL's (But it seems that currently Pinterest API only returns the original image)
• Implemented support to create a Pin using a Bitmap and updated the default sample to work with it.

First Beta Release - Silent login implementation

Finally I have managed to implement a proper silent login flow plus an application sample. The sample still has some bugs when updating the buttons, but only if the official Pinterest app is not installed on the user's device.