0.6.0

Lazy loading and better errors, courtesy of @sherbang in #22
Attachment download streaming, courtesy of @fundthmcalculus in #29
Various API model validation fixes, courtesy of @Yoshify

Note that 0.6.0 and 0.6.0.1 (version on PyPi) are the same version - a hiccup in the publish workflow required a new and unique version number, so I tacked on a .1 😅