Adds support for header_value for export header

[5tefan]

Summary

A potential solve for #947.

Adds a header_value property on Column and BoundColumn, analogous to the current header property. Then, adjusts as_values to use the header_value property for export column headers. The header_value property returns header for backwards compatibility, but provides a way to customize export headers independently of headers in the html table view. This is important because header may include custom HTML that shouldn't be included in export formats.

[jieter]

Looks good, I have added some suggestions.

[jieter]

I think we can safely write this as:

Suggested change

	column_header_value = self.column.header_value
	if column_header_value:
	return column_header_value
	return self.header
	return column_header_value or self.header

[jieter]

Suggested change

	customized using :meth:`~Column.render_value`.
	customized using :meth:`~Column.render_value`.

[jieter]

Suggested change

	For column headers, similarly sometimes :ref:Column.header`-may include html which
	For column headers, similarly sometimes :ref:Column.header`-may include HTML which

[jieter]

Isn't unicode a python 2 construct?

Suggested change

	:returns: `unicode` or `None`
	:returns: `str` or `None`

[mschoettle]

If this is used for exporting, would it make sense to have “export” in the argument name?

[jieter]

If this is used for exporting, would it make sense to have “export” in the argument name?

Yes, I think export_header is a better name, export_value instead of value would have been a better choice too, but it's too established to change it now.