Fix the fields_for method to include custom BootstrapForm options.

The FormBuilder.fields_for method only passes standard FormBuilder
options into the nested builder, causing the custom style, left, and
right options to reset on nested inputs.

This fix passes those options from the parent builder when using
fields_for while also allowing them to be overridden on the nested
fields.
Merge pull request #30 from awmichel/fix-fields-for-style-options

Fix for Horizontal/Inline Nested Fields