remove `private` from Wallet#client

Accounts class has a delegation on `client` to Wallet class, but
Wallet#client is private, thus will trigger the following exception
while executing `Wallet.accounts.new('abc')`

```
/Users/fleuria/.rvm/gems/ruby-2.0.0-p247/bundler/gems/ruby-wallet-edf7e2eb8a1b/lib/wallet/accounts.rb:5:in `client': private method `client' called for #<RubyWallet::Wallet:0x007fc79c55e9f0> (NoMethodError)
    from /Users/fleuria/.rvm/gems/ruby-2.0.0-p247/bundler/gems/ruby-wallet-edf7e2eb8a1b/lib/wallet/accounts.rb:13:in `initialize'
    from /Users/fleuria/.rvm/gems/ruby-2.0.0-p247/bundler/gems/ruby-wallet-edf7e2eb8a1b/lib/wallet/wallet.rb:21:in `new'
    from /Users/fleuria/.rvm/gems/ruby-2.0.0-p247/bundler/gems/ruby-wallet-edf7e2eb8a1b/lib/wallet/wallet.rb:21:in `accounts'
```