* Remove asset.validate_asset and move its validation checks to other areas
* Move Asset.get_asset_id to Transaction
* Remove Asset and AssetLink models
* Add test against creating TRANSFER transactions with bad asset
* Allow AssetLinks to be used in place of Assets in the Transaction Model and enforce `Transaction.transfer()` to only take an AssetLink
* Remove AssetLink's inheritance from Asset
* Remove id from the Asset model
* Fix get_txids_by_asset_id query for rethinkdb after removing asset's uuid
Because `CREATE` transactions don't have an asset that contains an id
anymore, one way to find all the transactions related to an asset is to
query the database twice: once for the `CREATE` transaction and another
for the `TRANSFER` transactions.
* Add TODO notice for vote test utils to be fixtures
* Update asset model documentation to reflect usage of transaction id
* Fix outdated asset description in transaction schema
* Fix typos pointed out in review by @ttmc
* Reword description of an input in the transaction schema
* Re-add removed comment in transaction model
* Fix small typos in some comments in the transaction model
* Add trailling commas to a multiline dict in the transaction model tests
* Fix small things with server docs changes
* Add description of public keys' association with conditions in transaction concept docs
* Reword description of Transaction.create's and args
* Rename output_uri to output_condition_uri
* Fix hardcoded class name vs. self.__class__
* Rename instances of 'out' with 'output'
* Rename instances of and with to avoid name clash with built-in
* Remove unnecessary renaming of cryptoconditions.Fulfillment import in transaction model
* Remove instances of in transaction model
* Remove usages of fulfillment in cases where input makes more sense
* Reword docstrings for init methods in transaction models
* Rename usages of condition where output is now a better fit
* Add descriptions to TransactionLink's txid and idx in schema
* Minor correction to output idx description in transaction yaml
* Add README.md to tests/ to describe test structure and pytest customizations
* Add package-level docstrings to backends
* Add README.md explaining the backend architecture
* Small improvements to docstrings in backend modules
* Restructure the backend automodule docs
* Add more docstrings to backend connections
* Add init to base backend Connection class to document expected interface
* Reword the backend/README.md docs
