v0.6.0.2

Support latest base64 package

0.6

• sendMail API was simplified (see Updating.md for details)
• Functions for graceful close were introduced.
• Use Text type instead of String internally.
• Use mime-mail.Address type instead of String for representing
  addresses. (Issues #78, #59)
• Introduce SMTP exceptions family and move to using that
  from fail method.
• Fixes possible double handle close in case of exceptions (Issue #76)
• SMTP: Fix issue with extra flushes. Now there are twice
  as few messages during communication and that largely improves
  communication speed.
• Module HaskellNet.SMTP.Internal was introduced. It contains
  internal functions that may be useful when implementing primitives.
  There is no API stability guarantees on the module.
• SMTP documentation largely improved
• Package moved from base64-string to base64 package (Issue #61)