[sldev] API for account balance

[Jason Giglio]

With all this web authentication overhaul and such, please keep in mind 
that a way to check our L$ balance with an HTTP API is sorely needed.

There's no way to validate that someone actually paid you right now.  A 
nice API that could be used from LSL or outside would be good.  It only 
needs to return the current L$ balance to help out a lot.

More information would of course be great, such as the last few 
transactions, but I'd happily settle for just balance.

-Jason