Rename getSlug to slug

another 2 % of #2124
- renamed `/ghost/api/v0.1/posts/getSlug/ to
`/ghost/api/v0.1/posts/slug/`
- renamed method getSlug to generateSlug
Fixes tag cache test issues introduced in c6c4d5a8
Merge pull request #2519 from halfdan/tag-cache

Fixes tag cache test issues introduced in c6c4d5a8
Merge pull request #2439 from sebgie/move-slug

Rename getSlug to slug
Adding database to bug report template
Update README & Contributing with details of stable branch