Feature debug storage

[localhuman]

Added debug storage for building and testing contracts that aren't deployed yet

When building and testing smart contracts from the prompt, it is often difficult to determine how a contract will act with differing states of storage. Currently, the only way to test persisted storage is to actually deploy an SC on a test network.

This change-set sets up a test LevelDB instance specifically for storing items persisted via the Storage.Put methods within build ../path/file.py test ... contract invocations, so subsequent calls to build .. test ... will be able to access storage items previously placed there.

This PR also adds a few things to the prompt interface:

• debugstorage on turn debug storage on
• debugstorage off turn debug storage off
• debugstorage reset resets the debug storage database

This PR also adds a property in neo.Settings.settings instances: USE_DEBUG_STORAGE which defaults to the value in your protocol.xxx.json

[coveralls]

Coverage decreased (-0.06%) to 68.256% when pulling 3138839 on feature-debug-storage into 8c13da7 on development.