Income taxes (Details 3) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Net operating loss carry-forwards	$ 25,425,715	$ 27,188,180
Stock options	3,482,800	3,514,890
Bonuses and salaries	26,993	33,111
Restricted stock	91,580	73,538
Provision for doubtful accounts	14,568	13,703
Foreign nonrefundable withholding tax	478,295	318,163
Foreign tax credit	335,698
Other		22,937
Total deferred tax assets	29,855,649	31,164,522
Less: Valuation allowance	(28,475,080)	(29,416,464)	(29,200,000)
Net deferred tax assets	1,380,569	1,748,058
Deferred tax liabilities:
Depreciation and amortization	(653,074)	(981,816)
Total deferred tax liabilities	(653,074)	(981,816)
Total deferred tax assets, net	$ 727,495	$ 766,242	$ 200,000