Finish TellorCaller.sol for mainnet

[0xsomewherecat]

Def of done:

1. Update TellorCallerMock.sol to reflect Tellor's spec change (the value's type changed from uint256 to bytes).
2. Implement Dependencies/TellorCaller.sol that is production ready to fetch real Tellor prices.

As for the 1st point, implement the type cast within the TellorCaller so that its client (PriceFeed) can receive the price as a numeric value.

Reference:
UsingTellor#getCurrentValue
https://github.com/tellor-io/usingtellor/blob/master/contracts/UsingTellor.sol