WIP: Custom vesting curves

[karzak]

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WIP:

• Added account type for periodic vesting. This enables arbitrary vesting schedules.
• Wrote tests in genaccounts and auth

TODO:

• Figure out how to handle the client for periodic vesting accounts. Inputing an array of VestingPeriods to the cli seems sub-optimal.
• Write tests in bank
• Refactor structure to reflect general consensus that vesting should be moved out of auth.

Open questions:
How should I organize this? I'm not sure what a stand alone module or sub-module should look like, given that vesting accounts are a pretty minimal extension of account functionality. See cosmos#4486

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