Bumped version to 0.6.4
Merge branch 'release/0.6.4'

 - Documentation of grimoire.api
 - Move grimoire.api.fs to clarify that it is imp'l details
Merge tag '0.6.4' into develop

Vist 0.6.4 upon the world

 - Document grimoire.api better
 - Clarify that what was grimoire.api.fs is really just implementation
 - details and is not for general use.