Html5 : Make it possible to specify the position of a figcaption.

fix #6
HTML5 : Make a_input_max and a_input_min take floats.
Use -annot and -bin-annot only if the major version of the current ocaml compiler is 4
Allow attribute required for select elements
Use List.append instead of @ in syntax extension
Optional argument for HTML5 attribute a_step
Merge pull request #13 from benozol/step-option

Minor improvements (and option for a_step)
Merge remote-tracking branch 'up/master'

Conflicts:
	lib/html5_f.ml
	lib/html5_sigs.mli