v0.6.0

By @xssnick:

• Wallet operations
• Cell signatures
• Mnemonic generation/parsing
• Store builder, cell conversions, new cell builder/loader methods
• Grams and Message small refactoring, added support for JSON serialization (suggested by @hotid)
• ADNL big bytes array fixed
• Big BOC serialization fixed

By @mr-tron:

• Added automatic TL [de]serializator