* Switched to lts-8.13, added custom-setup.
Our packages no longer elicit complaints like this:
Package tensorflow-foo uses a custom Cabal build, but does not use a
custom-setup stanza.
* Removed some extra-deps and explicit-setup-deps
* Removed provisions for different versions of stack lts.
We are now solidly in the 8-only territory.
Add support for logging to tensorboard
Based on @gnezdo's internal version with some differences:
* Uses a pure haskell implementation of EventWriter instead of FFI.
* Special `buildAnd*` functions were dropped in favor of using
`mergeAllSummaries :: Build SummaryTensor` with the normal
`build` function.
