Crunky REST API

We'll want Computable backend systems like Crunky to support a REST API. The goal of the REST API is to provide a standard way for data market users to communicate with backends. Here's a first cut at some proposed REST API options:

### Authentication
###### Description
- `AUTHENTICATE user-credentials`
- Allows a given user to authenticate itself to the backend
- Results of a spike outlining the Metamask authentication flow can be found [here](https://github.com/computablelabs/ld01/issues/16)

###### Endpoints
- `POST /signup` creates a new user record
- `POST /login` creates a new session record for a given user

### Marketplace Schema
###### Description
- `GET_SCHEMA auth_token`
- Returns the scheme of the dataset, likely in JSON format
- Can support different backend schemas (SQL/Spark/etc) with different JSON blogs

###### Endpoints
- `GET /marketplaces/:marketplace_id/schema` returns the schema for a data marketplace, given a marketplace ID

### Data Queries
###### Description
- `RUN_QUERY auth_token query-specification`
- Runs the specified query
- Security is a key focus here. It's likely that we'll have a number of fixed "whitelisted" queries rather than allowing for arbitrary queries to prevent SQL injection attacks or dataset scraping attempts.

###### Endpoints
- `POST /marketplaces/:marketplace_id/search` returns the result of the query defined in the request body

### Data Management
###### Description
- `ADD_DATAPOINT auth_token datapoint`
- Adds the specified datapoint from backend. One datapoint corresponds to one listing in market
- TODO: fix the terminology once we settle on it

###### Endpoints
- `POST /marketplaces/:marketplace_id/data` adds the associated data to the data marketplace, given a marketplace ID

###### Description
- `REMOVE_DATAPOINT auth_token datapoint`
- Removes the specified datapoint. One datapoint corresponds to one listing in market
- TODO: fix the datapoint/listing terminology later

###### Endpoints
- `DELETE /marketplaces/:marketplace_id/data/:data_id` removes the specified data from its data marketplace, given data and marketplace IDs

### Marketplace Discovery
###### Description
- `MARKETS_SUPPORTED`
- Returns a list of the markets this backend system serves and their deployed contract addresses
- No auth token needed since public information

###### Endpoints
- `GET /marketplaces` returns a list of supported data marketplaces

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Let's discuss the details of the REST API in the comments. As we gain clarity, I'll edit this top-level listing of the REST API to stay current.