Using error correction "H" where possible

[LoopingLudwig]

I did some bigger changes to use segno instead of qrcode to generate the QR codes, with the main goal to use micro QR codes. I expected that they will result in better recognition rate at small dimensions.
But after some, indeed not strictly methodical, testing it seems that the best in case of recognition is the use of normal QR codes with error correction "H".
Therefore I suggest to set also with qrcode the error correction level to "H", since in my understanding qrcode in contrast to segno is not using the maximal possible error correction level.
I didn't filed an other pull request, as the changes are quite big and i don't want to hijack your great project. But maybe my observations are of interest for you, or some one else.

If some one is interested here are my test results. The labels where printed at 1200x600dpi with no toner saving on an OKI MC332dn, which was also used for scanning.

scan Setting	scan lib	orientation	qrcode	segno QR	segno microQR EC „M“	segno microQR EC „Q“
100dpi grayscale	zbar	~30°	fail	fail	fail*	-
150dpi grayscale	zbar	~30°	fail	fail	fail*	-
300dpi grayscale	zbar	~30°	success	success	fail*	-
150dpi grayscale	zxing	~30°	fail	fail	success	-
100dpi grayscale	zxing	~30°	fail	fail	fail	-
100dpi grayscale	zxing	~0°	fail	fail	fail	-
150dpi grayscale	zxing	~0°	fail	success	fail	-
100dpi grayscale	zxing	~0°	-	0 / 2	-	1 / 2
150dpi grayscale	zxing	~0°	-	1 / 2	-	0 / 2
150dpi color	zxing	~0°	-	2 / 2	-	0 / 2
200dpi grayscale	zxing	~0°	-	2 / 2	-	1 / 2
300dpi grayscale	zxing	~0°	-	2 / 2	-	2 / 2

*Most probably micro QR codes are not supported by zbar