Note 7 - Warrants - Warrants Outstanding (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Balance, outstanding (in shares)	22,104,817	20,035,114
Balance, outstanding, price range (in dollars per share)
Issued (in shares)	2,611,513	219,298
Issued, price range (in dollars per share)		$ 0.70
Exercised (in shares)		(510,000)
Exercised, price range (in dollars per share)		$ 0.30
Balance, outstanding (in shares)	24,716,330	19,744,412
Balance, outstanding, price range (in dollars per share)
Minimum [Member]
Balance, outstanding, price range (in dollars per share)	0.125	0.125
Issued, price range (in dollars per share)	0.25
Exercised, price range (in dollars per share)
Balance, outstanding, price range (in dollars per share)	0.125	0.125
Maximum [Member]
Balance, outstanding, price range (in dollars per share)	1	1
Issued, price range (in dollars per share)	0.48
Exercised, price range (in dollars per share)
Balance, outstanding, price range (in dollars per share)	$ 1	$ 1