Readme Fixes and Improvements

Identical to PR #1655, but with the commits squashed into one.

These are mostly some edits to clean up inconsistencies in the Readme.

Rationale for the edits:

Switched places of Wii and Triforce in lists. Triforce emulation is pretty obviously not treated with the same importance as GameCube and Wii emulation is and so I feel it makes sense to have it listed last rather than second.

Made the spacing between slashes consistent. In some locations slashes had spaces and in others they didn't. I chose to add spaces throughout as it makes it a bit easier to read.

Gave Linux and OS X separate lines in the system requirements. If Windows gets its own line it makes sense for Linux and OS X to also have their own lines. This also clears up a potential ambiguity in what 10.9 is referring to.

Unified title capitalization. Some titles used capitals for the whole title and others only used capitals for the first word. I decided to capitalize all the titles.

Removed Vista or higher as higher as a requirement for using D3D. As Vista or higher is already a requirement for running Dolphin on Windows this is completely redundant. Also cleaned up a bit of a typo here.
Dolphin doesn't support Windows XP at all anymore. It's mentioned in the system requirements near the top of the readme.