Summary of significant accounting policies (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock-based expense and assumptions
Total stock-based expense	$ 1,135,047	$ 570,810	$ 829,733
Stock options
Stock-based expense and assumptions
Total stock-based expense	1,135,047	555,882	739,992
Unamortized stock-based expense	1,383,725	786,668
Weighted average remaining amortization period	1 year 9 months 18 days	1 year 8 months 8 days
Capitalized stock-based compensation expense	100,000
Warrants
Stock-based expense and assumptions
Total stock-based expense		14,928	89,741
Processing and service costs
Stock-based expense and assumptions
Total stock-based expense	262,775	135,444	211,582
Selling, general and administrative expenses
Stock-based expense and assumptions
Total stock-based expense	$ 872,272	$ 435,366	$ 618,151