Note 5 - Share-based Compensation - Stock Options Issued Outside of the 2007 Equity Incentive Plan (Details) - Outside of the 2007 Plan [Member] - $ / shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Options outstanding, beginning balance (in shares)	20,148,766	19,394,975
Weighted average price per share, beginning balance (in dollars per share)	$ 0.40	$ 0.40
Options outstanding, granted (in shares)	583,526	263,523
Granted, exercise price per share (in dollars per share)		$ 0.33
Weighted average price per share, granted (in dollars per share)	$ 0.59	$ 0.33
Options outstanding, expired (in shares)
Weighted average price per share, expired (in dollars per share)
Options outstanding, ending balance (in shares)	20,732,292	19,658,498
Weighted average price per share, ending balance (in dollars per share)	$ 0.41	$ 0.40
Minimum [Member]
Exercise price per share, beginning balance (in dollars per share)	0.18	0.18
Granted, exercise price per share (in dollars per share)	0.50
Exercise price per share, ending balance (in dollars per share)	0.18	0.18
Maximum [Member]
Exercise price per share, beginning balance (in dollars per share)	1	1
Granted, exercise price per share (in dollars per share)	0.67
Exercise price per share, ending balance (in dollars per share)	$ 1	$ 1