Adds MemoriesAdapter and layout memory_list_item

This is a first step towards a list of stored memories.
Removes the linked account verification from LinkDropboxFragment

It now assumes that when the LinkDropboxFragment is displayed there's no already linked account.
The MainActivity is going to manage this.
Adds the linked Dropbox account verification to MainActivity

This way the MainActivity decides when the LinkDropboxFragment is showed or not.
Removes the DropboxAuthManager param from the onDropboxAccountLinked method on OnDropboxAccountLinkedListener interface.

Since the MainActivity is dealing directly with DropboxAuthManager it's not necessary to return a new object
Makes LinkDropboxFragment retrieve a DropboxAuthManager onAttach
Makes DropboxAuthManager able to start the authentication process from a different context

Instead of tying the DropboxAuthManager to a context forever, now it's able to use a different context
for authentication process, the current context (Activity or Fragment) is not altered
Changes LinkDropboxFragment to use itself as a context to Dropbox authentication process
Makes minor javadoc improvements on MemoryModel
Adds DropboxDataManager

An abstraction layer to be used when acessing Dropbox data
Adds sync() method to DropboxAuthManager
Makes MainActivity load and display all the memories stored on Dropbox after authentication.
Merge branch 'memories_list' into dropbox_features