Switched Asset Serialization to Force Text

• Switched the “Project Settings › Editor › Asset Serialization” options to “Force Text”.
	• This auto-converted all existing `*.unity`, `*.prefab`, `*.mat`, `*.physicmaterial`, `*.anim`, and `*.cubemap` asset files.
	• It also auto-converted all of the `ProjectSettings/*.asset`s.
Added fairly-standard Unity project .gitignore

• Added .gitignore for Unity projects, copied from GitHub's standard ignores (at https://github.com/github/gitignore/blob/master/Unity.gitignore).
Removed the `.unitypackage`s.

• Deleted the `Packages/*.unitypackage`s.
	» unitypackages are a PitA unless you keep them up-to-date with every commit.  They can be added for tagged releases.
	» They're also redundant.  If they were set up to be auto-built (like how LESS auto-builds in to CSS) that would be fine, but they don't.
	» And Unity doesn't have a ubiquitous solution to make sure they're always auto-built— one would either need to ensure everyone is using Unity Pro, or has a git pre-commit hook set up, or leaves a command-line script running.