Add stacks endpoints and refactor for consistency

[whoabuddy]

Adds:

• get BNS name for a Stacks address /stacks/get-bns-name/:address
• get STX balance for a Stacks address /stacks/get-stx-balance/:address

NOTE: the STX balance is in uSTX where 1 STX = 1,000,000 uSTX

BREAKING CHANGE: this changes the /stacks-block-height endpoint to /stacks/get-block-height to provide consistent naming across all directories. It's updated in the README and on the landing page as well.