[TEST] payment test suite work

- add tests for `tec` errors
- re-implement duplicate client resource id in payments-test
- fix cases where re-using of hashes would result in hanging transactions
- add generate hash method to testutils
- add validated responses for `tes` and `tec` errors
- move testutils exports to bottom block
[TASK] rename _payment-test to _payments-test
Save unsubmitted transactions to database
Merge pull request #203 from ripple/save-unsubmitted

[TASK] Save unsubmitted transactions to database
[TASK] bump ripple-lib to 0.9.3-rc2
[TEST] cleanup payment test

- remove unnecessary finalized rippled responses
- add catch for errors on duplicate resource id test
Merge pull request #202 from geertweening/fix/payment_test

[TEST] payment test suite improvements
[FEATURE] add paging to GET /balances
[DOC] update documentation for payments API endpoints
[FEATURE] add paging to GET /trustlines
Merge pull request #205 from boxbag/account-lines-paging

[FEATURE] add paging to GET /trustlines and GET /balances