Wk issue #81 methods clean up (#100)

Fixes for issue #81 
* Changes:
* move tests from proposal.cpp to proposal_tests.cpp, now all test are int chain_tests,
* delete template specjalization proposal_database_fixture_for_plugin.

* Changes:
* use vest instead of transfer_vest,
* deletion of transfer_vest.

* User built-in transfer metode.

* Remove of prepare_plugin.

* Change name of proposal_database_fixture to sps_proposal_database_fixture. Remove CRTP idiom.

* Changes:
* remove try-catches from list_proposal, list_voter_propsoals, find_proposals from sps_proposal_database_fixture,
* add more unittest.

* Add FC_CAPTURE_AND_RETHROW