minor fixes for sharekit

[eric-sofisoftwarellc]

There are three small changes:

• The current master head doesn't compile due to missing evernote #defines in the config file. I added empty ones.
• #define SHKSharedWithSignature appears twice in the config file
• SHKTwitter.m validate checks for status.length >= 0, but should check for status.length > 0.

Thanks!