core - implement radd for 0 so sum works correctly (and allow 0 elsewhere for consistency)

Ignore-this: 51fcc7573054862dc40c22e93cdaef21

darcs-hash:20130927032649-3a4db-a659a3c29d442ff516e6fc6e437ca154dbd39b78
(core - add explicit test for sum)

Ignore-this: cc7af7caa1e9a57560f5022bcf8e0b94

darcs-hash:20130927033320-3a4db-d0a712a8da41112b592c3975ce543703cc39c5f9
misc - use platform-specific default integer dtype for indices

Ignore-this: da52658caef8ebb1a11ac403fe2fe61b

darcs-hash:20130928011706-3a4db-890ef03f29a671d9558ede3d87122dfe02476573
(README - minor improvements)

Ignore-this: 165032367606e5f8a03efc5c22aedb1c

darcs-hash:20130928011855-3a4db-3fa80d2264b3c0ba734dc131ef34fc42735142a1