Implement the submit mode for getblocktemplate

Add the missing processing for the "submit" mode of getblocktemplate. This is required for stratum servers that do not use submitblock for some reason.

Note: I have also a version with line 400 being: strMode = "submit";
But it does not seem to be required.
Update rpcmining.cpp

Removed extraneous return.
Merge pull request #1 from danbi/patch-1

Implement the submit mode of getblocktemplate.