(WIP) Integrate encryption and decryption

[hanwencheng]

resolve #3

• Integrate encryption and decryption on iOS

  • update the development environment to latest
  • Implement basic encryption and decryption function
  • Refactor committed code with objective-C conventions ( helps are welcome, I am new to objective-c programming )

• Integrate encryption and decryption on Android

  • Implement KeyPairGenerate function for encryption, (save key into Keychain.)
  • Implement basic encryption and decryption function
  • Save the encryted ecc keys into SharedPreference

[mvayngrib]

@hanwencheng thanks for the PR!

question: are there some existing tests / fixtures for ECIES that you can adapt to test your implementation? For example, these repos have some tests, but maybe there are some standard fixtures somewhere:

• https://github.com/bitpay/bitcore-ecies
• https://github.com/bitchan/eccrypto

this would have the added benefit of demonstrating compatibility with other ECIES implementations (which a counterparty might be running)

edit: btw, i don't mean u need to create some special testing environment. It's fine if i can run them in an app by doing require('react-native-ecc/test')