Migrations

[Art3miX]

Added migrations to all contracts.
All of them can be called optionally from the covenant contract, which should be the admin of all other contracts.

Everything is optional, so you can send msg to covenant to only pause clock:

{"update_config" : {
    "clock" : { "pause" : {} }
}}

Update withdrawer on holder contract:

{"update_config" : {
    "holder" : { "update_withdrawer" : { "withdrawer": "some_withdrawer" } }
}}

Or both at the same time:

{"update_config" : {
    "clock" : { "pause" : {} },
    "holder" : { "update_withdrawer" : { "withdrawer": "some_withdrawer" } }
}}

[bekauz]

lgtm. let's test this as soon as this and #27 are merged.