Sidenav tree cleanup

[snide]

Replaces this PR #112

Closes #50 and https://github.com/elastic/Design/issues/647

This is a WIP Sidenav exploration. It needs some work on the React side before we can merge it, assuming people are OK with the style.

Changes the following:

• EuiSideNavGroup is a new component for nesting a group of EuiSideNavItems. It adds some borders and ticking to items underneath. It can be nested indefinitely (as space allows)
• EuiSideNavItem now has a parent prop. This will render some arrow carets and other styling.
• Removing the EuiTitle inheritance in EuiSubNavTitle and just applied those styles directly in the CSS. Make the component more flexible so you can include icons and the like.

Stuff to do before merge

• Play with focus state. It's a little weak.
• Chat with CJ about how to pass icons in to the cloned EuiSideNavItem element.
• Better document the new props / usage.
• Fix sidenav in panel.

Need help on the React side (will be a later PR)

• EuiSideNavGroup needs to wrap around subnav items in our actual documentation sidenav. I'll bug @cjcenizal about it, likely easy. Add back SideNav submenu in documentation #116
• With the nesting, my guess is it makes sense to make an all-in-one component where you just pass in a JSON tree and have it do all the component construction for you. That's probably beyond my "quick" abilities. Make SideNav compose itself in JS #115

[snide]

@cjcenizal for now i've simply commented out the side nested items in the docs. We can add them back in with the new formatting later.

[snide]

@formgeist I was able to apply the correct padding on the title icon versions in this PR. Should look pretty close to yours, minus the arrow coloring (which I'll address in a later PR).

[cjcenizal]

Nice work! LGTM. I'll take a whack at this once it's merged.