Section objects should align themselves on write

[mjhouse]

Section struct instances (#29) should have access to the alignment of their data via the sh_addralign attribute on the SectionHeader. As part of this issue, a method with the following signature should be added to the Section struct:

pub fn align(&mut self);

This function will check the sh_addralign attribute and pad the body of the section with zeroes until it meets the alignment constraint. It should be used by the Binary struct (#24) just before the section data is written.

This issue relies on #29.