Implement cancel functionality in THTTPClient

Change-Id: Id3da4e9dbb1259224c5623f52c8074c1262f4b12
Put a note preview web view in 'Save to Evernote' view controller

Change-Id: Ie1b7fac1a589c3a0604364c663ca133a793e4c03
Support NSURL in ENSaveToEvernoteActivity

Change-Id: Id25c5df9d4b0bce6cc83d66abed0b44d9e41b97a
Support PDF display
Improve PDF display support
fix css button color