Update:
We have implemented an option to sign transactions with a help of MetaMask wallet, providing a secure, neutral, third-party place for our users to sign their transactions. MetaMask keys are stored on the user’s own browser, not on any remote servers. This gives the user more control over their public and private keys.

Learn more about the MetaMask plugin: https://kb.myetherwallet.com/migration/moving-from-private-key-to-metamask.html