unify: change format for assigning blocks

Its now of the form:

POST /v1.0/vaults/{vault_id}/files/{file_id}
body:

[
    ["fecfd28bbc9345891a66d7c1b8ff46e60192d284","0"],
    ["63cac3ed731e4b1f868cb3adaef591464de54a67","100"],
    ["a6f76d7c3b9e2576fa74c5dc438304bcdf3247d0","200"]
]
efficiency: use single list comprehensions

- Addresses Zhihao's comments
- the unpacking from the payload is now pythonic.
    block_id, offset = mapping
- use single list comprehensions for setting both offset and blockid in
  a list of lists
Merge pull request #170 from TheSriram/unify_formats

unify: change format for assigning blocks