Add erb templates; move WebApi; create portfolios on before filter
Fix DCell::Server::InvalidMessageError

The error "invalid message: undefined class/module StockExchangeSim::" was
due to missing knowledge of celluloid about StockExchangeSim::Portfolio
and StockExchangeSim::Transaction objects.

Note that the error doesn't trigger while handling StockExchangeSim::Order.

This topic needs further investigations.