Adjust color of company name

I think company name should be more emphasized. However, an experiment
with our accent color doesn't look right to me. So I tried black 54%
(which is an alternative suggested by the doc). It looks good.
Improve shadows

_ Every clickable object will have shadow in normal state. This encourage
users to interact with them. Also, the shadows create a natural feeling
of a sheet of paper being placed on top of another sheet.

_ Two-layer shadow is used: One layer for depth and another for definition
Improve colors

_ It should be better to maintain element's basic physical properties upon
the lifting effect. For example, when a sheet of paper is lifted up,
a slight change in shadow is natural, but changing to a completely different
background color is not. The top 6 cards are edited because of this.

_ Similarly, the color of buttons will no longer change upon lifting. This is
also consistent with buttons in the Polymer project.
Resize the 4 cards at the bottom

These four cards span across the bottom 'sheet', effectively splitting the
sheet into two parts.

Resizing the cards so that they're contained within the bottom sheet leads
to a more natural and logical look.
add rounded corner for all rectangle elements
add some spacing between the 4 project cards
reduce rounding radius
adjust shadows to standard level in the doc
Merge pull request #1 from npvn/phuc

Landing page UI improvements