Integrate tellor prices into the api

[skuhlmann]

this pr is in tandem with the front end repo pr with the intent to get tellor onto oracles.club
dragonfly-xyz/oracles.club#10

This pr includes:

models/tellor.py

• eth and btc tables

tellor_methods.py

• all methods matching the pattern of the other oracles
• wraps etherscan to get eth and btc price into database

oracles.py

• implements tellor exactly as other oracles are implemented

Let me know if there are questions/comments/changes/ect...

Thanks

[ashwinrz]

Looks great - thanks for the contribution! Will get this merged as soon as we can