Add support for 0.12

[ruimarinho]

Differences from 0.11:

• Username-only authentication is no longer possible without read access to the random cookie file generated by bitcoind when -rpcpassword is not set (see https://github.com/bitcoin/bitcoin/pull/6388/files). This is the output of bitcoin-cli when connecting without -rpcpassword and without access to the cookie file:

  error: Could not locate RPC credentials. No authentication cookie could be found, and no rpcpassword is set in the configuration file (/root/.bitcoin/bitcoin.conf)
  

  Technically, it is possible to send a request via Basic Authentication by reading the cookie file on the server and using the predefined username __cookie__, although that defeats the purpose of not having a password set in the first place.

  Support for sending requests with username-only will remain while 0.11 is maintained.

• Some HTTP errors, such as 401 Unauthorized no longer rely on a message on the body. Instead, a proper status code and message are set.

• HTTPS has been removed. Instead, stunnel or other similar techniques must be used instead. Support for sending HTTPS requests will remain while 0.11 is maintained.

• The server response header is no longer available.

• The mediantime property was added to the getBlockByHash() call.

• The size property was added to the getTransactionByHash() call.

• The getMemoryPoolContent() and getMemoryPoolInformation() calls no longer return 404 when no information is available. Instead, they now return a proper empty object.