CredentialString(key string) can not deal with multi-layer keys

[xiujiao]

CredentialString(key string) only can get the first layer string values. It will be nice we can run Credential(key) to get nested credential with any type. for example Credential(protocols.amqp.uri)will get amqp://guest:guest@rabbitmq:5672,Credential(protocols.amqp.ssl) will get false

"VCAP_SERVICES": {
  "p-rabbitmq": [
   {
    "credentials": {
     "protocols": {
      "amqp": {
       "uri": "amqp://guest:guest@rabbitmq:5672"
       "ssl": false
      } }  } }]
}

[xiujiao]

I submit a PR to solve this issue. #24
Could you please take a look? @joefitzgerald @krishicks @ArthurHlt
Thank you very much