Allow percentage grid sizes via inner element

[renierr]

Masonry allow to define the grid size via an inner element (first).
this should be done if columnWidth is set to 0

An inner Element will be created with a special style classname "masonry-grid-sizer" where you can specify a width (also percentage width)

[renierr]

see: https://github.com/renierr/Masonry/commit/b848c0d62cca627944bde323d93ce972dbab942e

[alump]

Thanks for the patch... I will use it as base (I have already some of these changes).

One change I need to do is to hook layout size changes, so relative sizes will be updated with layout size.