Issue #1865 - Clean up RPC help messages

Based on the proposal, update the help message of rpc methods
- strings arguments are in double quotes rather than square brackets
- numeric arguments have no quotes (and no default value)
- optional parameters are surrounded by round brackets
- json arguments are strings but don't use double quotes

Added 3 sections for the details
- Arguments: lists each argument, it's type, required or not, a default, and a description
- Result: The method result, with json format if applicable, type, and a description
- Examples: examples calls using bitcoin-cli and curl for json rpc call

Problems
- maybe this is too verbose
- lines might be too long
- description are not good or complete
- examples may be too much

src/bitcoinrpc.h

@@ -153,6 +153,9 @@ extern json_spirit::Value ValueFromAmount(int64_t amount);
 extern double GetDifficulty(const CBlockIndex* blockindex = NULL);
 extern std::string HexBits(unsigned int nBits);
 extern std::string HelpRequiringPassphrase();
+extern std::string HelpExampleCli(std::string methodname, std::string args);
+extern std::string HelpExampleRpc(std::string methodname, std::string args);
+
 extern void EnsureWalletIsUnlocked();
 
 extern json_spirit::Value getconnectioncount(const json_spirit::Array& params, bool fHelp); // in rpcnet.cpp