v2.0.0

 - [API Changes] - Return undef if there's no temp, powered on or power
   cycle attribute - Removed smart_is_available method, availability will
   be automatically checked on instantiation and an exception thrown if
   unavailable. There's nothing useful to do if SMART is not available
   anyway.
 - [Fixes] - Use unsigned int for uint64_t
 - [Other] - Reorganised and updated docs - Refactored XS having actually
   read some XS docs  - Use namespace::clean - Use predicate for checking
   if SMART data has been read, use before modifier to reduce repitition of
   checks
Adding README