[FIX] cap IOU Amounts to their max and min value

respect rippled's limits
[FIX] from_human 'XRP' with full name

from_human 'XRP - Ripples' should result in native XRP
[FEATURE] add paging account request

some requests return results that can be paged through, e.g. `account_lines`
use `limit` and `marker` options to specify results per response and position

change `requestAccountLines` and `requestAccountOffers` to use the `pagingAccountRequest`
[FIX] get interest for currency without interest

- `get_interest_at` for a currency without interest would not hit the intended check since the function was used as variable instead of calling the method
[FIX] Currency constructor with Currency object

clone the given Currency instance in `parse_json`
[TEST] add missing testcases for interest bearing currencies

- `get_interest_at` by providing a Date object
- `get_interest_at` for a Currency without interest
Merge pull request #199 from geertweening/fix/amount_cap

[FIX] cap IOU Amounts to their max and min value
Merge pull request #201 from geertweening/fix/from_human_full_name_native

[FIX] from_human 'XRP' with full name
Merge pull request #202 from geertweening/feature/paging_account_request

[FEATURE] add paging account request