API calls to work with raw objects

[Kleshni]

I think current API lacks 4 very useful methods:

• To list available object hashes for some criteria such as type, age, extra POW amount, some monotonous counter (like "rowid" from SQLite, possibly encrypted to hide internals from the API user).

• To get an object by its hash.

• To send a prepaired object. Current API has two similar methods, but they are undocumented and don't allow me to send objects of arbitrary type, only messages and public keys.

• Like previous, but with already calculated POW.

Their absense prevents using Bitmessage as a base for different protocols and extensions.

I can code them and make a pull request.

[PeterSurda]

Ok, feel free to contribute, please consult the Contribution guidelines and ask here or on gitter if you have questions.

[dimyme]

using my fixes adding protocols and such is easier in --curses mode

I already made "menu 9" availbale for such ideas