Update MDLDocument, add centralized MendeleySDK header file

Add MendeleySDK header
Add read, starred, volume, pages properties to MDLDocument
Add mark as read, mark as starred methods to MDLDocument
Add issue, addedDate properties to MDLDocument
Update MDLDocument with writable inUserLibrary property

Fix nil authentication failure callback bug
Add method to reset shared client instance

Fix retained notification observer for multiple authentications
Clean code
Add folder identifier to MDLDocument
Add volume and deletionPending properties to MDLDocument
Update MDLMendeleyAPIClient with public authentication method
Update MDLDocument with keywords and tags
Update MDLDocument to store keywords and tags as arrays of strings
Update MDLDocument with moveToTrash method
Update MDLDocument to fetch all detailed properties

Add missing properties to MDLDocument
Add identifiers property to MDLDocument, close #4
Fix typo (numberOfDocmuents), close #3
Improve podspec dependencies declaration
Refactor string constants naming convention
Update MDLUser with contact properties
Fix MDLDocument details formatting with missing data
Add support for public files

Update MDLFile to add publicURL property
Update MDLDocument to parse public files in document details
Update MDLDocument details fetching to infer DOI from identifiers dictionary
Fix NSURL class for MDLFile public URL