Implement bip39 account generation - Closes #133

[alepop]

• Add mnemonic module
• Add API method for generating new Account

In this implementation, I use some code from lisk-nano for generating a random seed. And take functionality from bitcore-mnemonic package. Why don't use bitcore-mnemonic package instead?
Because there too many dependencies.
Close #133

└─┬ bitcore-mnemonic@1.2.5
  └─┬ bitcore-lib@0.14.0
    ├── bn.js@2.0.4
    ├── bs58@2.0.0
    ├── buffer-compare@1.0.0
    ├─┬ elliptic@3.0.3
    │ ├── bn.js@2.0.4  deduped
    │ ├── brorand@1.0.5
    │ ├─┬ hash.js@1.0.3
    │ │ └── inherits@2.0.1  deduped
    │ └── inherits@2.0.1  deduped
    ├── inherits@2.0.1
    └── lodash@3.10.1

[coveralls]

Coverage increased (+0.06%) to 99.235% when pulling 48851b1 on alepop:implement-bip39-account-generation into 3107ff7 on LiskHQ:development.

[coveralls]

Coverage increased (+0.06%) to 99.235% when pulling dd23ce8 on alepop:implement-bip39-account-generation into 3107ff7 on LiskHQ:development.

[toschdev]

Please use the cryptographic function instead of an API call:
https://github.com/LiskHQ/lisk-js/blob/development/lib/transactions/crypto/keys.js

It is also synchronous and already tested

[toschdev]

The callback is not necessary, as stated the function is synchronous, you can just return it.

[alepop]

But all API methods receive the callback as the second parameter and return this and I follow that design.

[coveralls]

Coverage increased (+0.06%) to 99.236% when pulling 6594836 on alepop:implement-bip39-account-generation into 3107ff7 on LiskHQ:development.