Compact semantic_form_for class array

If the FormHelper.default_form_class is set to nil then we get extra spaces in the class in the output HTML. This will compact the classes array and knock those pesky nils right out.
Adding rspec for compacting the classes on the semantic_form_for helper
Fix to datetime_picker to format the value compliant to standard.
[fixes #942]
Merge pull request #943 from craigjackson/datetime_picker_value

Fix to datetime_picker to format the value compliant to standard. [fixes #942]
Allow for html options within check_box collection
Merge pull request #944 from rabidpraxis/check_box_collection_html_options

Allow for html options within check_box collection
Merge pull request #929 from oldbonsai/patch-1

Compact semantic_form_for class array
Simplify README example

Closes #940