Update .placeholder() mixin call

In less/mixins.less this mixin is being created with @placeholderText
color as default, which is actually set as @placeholderText: @grayLight
in less/variables.less so it's redundant to make call like this:

.placeholder(@grayLight);

make responsive input fields not wrap for fluid input prepend/append by setting block back to inline-block (oversight of 2.0.3)

No need to specify units when a value is 0, thanks to RECESS