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Accounts receivable (Details 1) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Opening allowance for doubtful accounts $ 193,338 $ 223,043  
Provision for doubtful accounts 0 0 $ 32,933
Accounts written off during the year 0 (29,705)  
Ending allowance for doubtful accounts $ 193,338 $ 193,338 $ 223,043