Net income per share (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Net income	$ 1,282,723	$ 1,875,328	$ 3,043,028	$ 3,586,954
Amounts allocated to participating preferred stockholders under the two-class method	(110,402)	(214,905)	(261,909)	(411,051)
Net income applicable to common stockholders (basic and dilutive)	$ 1,172,321	$ 1,660,423	$ 2,781,119	$ 3,175,903
Denominator:
Weighted-average common stock outstanding (basic) (in shares)	49,602,206	53,082,296	50,186,828	53,439,467
Common equivalent shares from options and warrants to purchase common stock (in shares)	2,385,489	748,238	2,214,962	651,002
Weighted average common stock outstanding (diluted) (in shares)	51,987,695	53,830,534	52,401,790	54,090,469
Basic net income per share applicable to common stockholders ( in dollars per share)	$ 0.02	$ 0.03	$ 0.06	$ 0.06
Diluted net income per share applicable to common stockholders (in dollars per share)	$ 0.02	$ 0.03	$ 0.05	$ 0.06
Total anti-dilutive securities (in shares)	4,960,758	11,673,467	6,637,513	12,890,660
Contingently issued restricted shares
Denominator:
Total anti-dilutive securities (in shares)			396,500	718,407