Review all APIs, add new optional parameters and change some old parameters to optional

[abitmore]

User Story
We'll soon support optional parameters for APIs (bitshares/bitshares-fc#126), so we can update existing APIs to be more user-friendly.

For example, get_call_orders API now has two parameters, asset and limit, a user have to specify a limit, and can only query for the first page. If we change limit to optional and have a default value for it, users can simply call get_call_orders(asset) to fetch the top results; if we add an optional start, then users will be able to fetch next pages.

CORE TEAM TASK LIST

• Evaluate / Prioritize Feature Request
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation