Number Precision - Use Repo.CurrencyValue to represent amounts uniformly

[amangale]

This PR makes the changes required for bringing in repo.CurrencyValue to represent amount/value uniformly across the code.
All the wallet interface methods have also been updated to use CurrencyValue. All the wallet implementations also have been updated.

[coveralls]

Coverage decreased (-0.4%) to 38.091% when pulling c080927 on numprec into e225593 on master.

[drwasho]

I think it would be useful to define some requirements for code review and the merge conditions; I'll check off what we have confirmed:

• 1. Passing unit tests
  • Determine if existing unit tests cover the new code?
  • New and/or updated unit tests written (if applicable)
• 2. Passing QA tests
  • Determine if existing QA tests cover the new code?
  • New and/or updated QA tests written (if applicable)
• 3. Passing functional tests (on a new node, not an 'upgraded node')
  • API calls as per expected requests/responses
    • GET /wallet/balance
    • POST /ob/listing
    • POST /ob/purchase
    • POST /ob/orderspend
    • POST /ob/orderfulfillment
    • POST /ob/ordercompletion
    • GET /ob/order/:orderId
  • Create and edit listings
  • Test buying and selling a listing
  • Test various order states +/- disputes
• 4. Passes code quality review
  • Metalinter
  • Error and state handling to avoid panics
  • Comments
• 5. Passes acceptance criteria (TBD)

[amangale]

@placer14 @cpacia -
I have added a sample request - response transcript here: https://gist.github.com/amangale/2d91c8dc4eb95a1fdf34363b83cf901d

This is indicative of the changed responses due to the repo.CurrencyValue introduction.

[placer14]

For others following along, we're going to stick a pin on this PR and merge it into a master-derived branch (called ethereum-master) so we can continue refining this work without blocking this request further.