Update readme. Close go-bongo#6

Diff tracker bombed out when it reached a nil pointer - fixed.

Slight modification to diff tracker to use String() with structs when…

… available

Added SetOriginal to difftracker, a few other minor tweaks.

Exposed getChangedFields

Split out validation and pre save hooks into PreSave so we can use th…

…at separately for "fake save"

Close go-bongo#3, stabilized validation + delete

Merge branch 'master' of github.com:maxwellhealth/bongo

Fixed readme with cascade

Plug for goconvey in readme