Input validation

[LeoPatOZ]

Resolves #50

tldr:

• Adds validation to max_block_range (asserts > 0) on all scanners
• Asserts count in latest scanner > 0
• Validation to to / from block in historic mode. Neither can be bigger that latest block
• In order to achieve this more neatly we expose an shared internal build method. This way the api remains untouched and connect can be used for validation when needed.

[0xNeshi]

Close to merging!

[0xNeshi]

Good job!