NIfTI-2 Support

[liningpan]

This PR is based on #90 , and resolved most issues raised in the original PR, including getters / Into&From traits. The PR also resolved several merge conflicts with current master.

Regarding using f32 for nifti1 when every possible, I don't think it's quite possible with the current NiftiHeader design

pub enum NiftiHeader {
    /// The underlying header version is `NIFTI-1`.
    Nifti1Header(Nifti1Header),
    /// The underlying header version is `NIFTI-2`.
    Nifti2Header(Nifti2Header),
}

We might need to make NiftiHeader a trait with a floating number type defined separately for Nifti1 and Nifti2

[nilgoyette]

Thank you @liningpan. I started reviewing but I stopped because I'm curious about @benkay86 opinion. Is this what you had in mind?

If I may ask, do you guys know each other? Why did you continue his work?