Change /wallet/encrypt to /wallet/init

[DavidVorick]

'encrypt' is a bit of a confusing term, since the call can only be made once and it's really initializing the wallet.

'init' is a better name.

related files:

doc/API.md
api/api.go
api/wallet.go

compatibility should be preserved, such that /wallet/encrypt still works.

[DavidVorick]

A 'siac' command will also need to be updated. If not updated in this issue, a separate issue can be opened to have that resolved.

[DavidVorick]

fixed by #754