Several improvements

Account
* refactoring of  init
* Doku improved
Asset
* AssetNotFound handling improved
* Operation for equal and unqual added
Price
* usage of the new equal operation from Asset
Steem
* Improve key handling in account creation
Storage
* add sqlite3_copy and recover_with_latest_backup
beemgraphenebase/account
* add get_blind_private, get_public_key, get_secret, derive_private_key, child and derive_from_seed
Unit tests
* Use setUpClass to speed unit tests up (Steem is now created at the begging auf each unit test class
* Add unit tests for asset
* Add unit test for new beemgraphenebase/account functions