Generalisation: using "Avatar of" instead of "Gravatar for"

Follow-up to pull request #35 (see https://github.com/gae-init/gae-init/pull/35 for rationale)
Fixed typo
Generalisation: using "Avatar of" instead of "Gravatar photo of"

Using "Avatar of" instead of "Gravatar photo of" so that someone may change it to another provider or a custom solution to get such avatars (and who says it needs to be a photo?)
Merge pull request #35 from mdxs/patch-2

Generalisation: using "Avatar of" instead of "Gravatar photo of"
Merge pull request #36 from mdxs/patch-3

Generalisation: using "Avatar of" instead of "Gravatar for"