Rename "Staking" to "Backing" in the rainbow contract

[alexprate]

The mechanism as described by Rieki in his rainbow paper and as developed by Chuck is more of a backing mechanism. The classic staking - as in locking up a token for a period of time doesn't yet exist in the contract. It might be confusing to keep the terminology Staking for what is more of a backing.

[alexprate]

@chuck-h I'm in favour of updating the name from staking to backing to avoid confusion.

[alexprate]

@chuck-h It was agreed during the last CWG to rename Staking to backing to avoid confusion - at a minimum on the UI and if possible at the contract level.

[chuck-h]

code changes done in PR #460 . (Video tutorials remain using "staking" at this time.)