Add "Migrating from Ethereum" guide and example contracts

Problem:

Newcomers from Ethereum usually have troubles understanding Tezos architecture and LIGO coding best practices.

Solution:

1. Provide a document describing inherent differences between LIGO and Solidity, and Tezos and Ethereum.
2. Add complementary example contracts
3. Use truffle library known to Ethereum devs to write tests for contracts

Known issue:

Currently one of the migrations does not go through, probably due to a bug in truffle for Tezos. Replacing { value: 4, fn: null } with { fn: null }, { value: 4 } in 5_deploy_lambda_in_storage.js solves the issue but it just works this way in this particular case – other migrations for contracts with records in storage do not require passing record items as separate parameters (see, e.g., 1_initial_migration.js).