Well, it had to happen some time: Bitcoin is crashing. The price of the crypto-currency tumbled nearly 21 percent on Friday to $877.46 and has fallen more than 29 percent since closing at a record high of $1,237.96 on Wednesday, according to Bitcoin data tracker BitcoinCharts.com. (Story continues after chart, courtesy of bitcoin.clarkmoody.com.) Much of Bitcoins meteoric rise in the past few months has been due to speculative trading in China, the Wall Street Journal pointed out earlier this week (subscription only). …