Simplify parsing by using more slices

Closes #7
Fix error messages to match assertions

Closes #8
Show that numbers larger than uint64 cannot be parsed

Closes #9
Simplify Scan() slightly

Closes #10
Push to version 2.0.0

Breaking changes:
Changed from pointers to value arguments.
If this breaks your code, use latest stable v1 (tag:v1.1.0).
You could also use gopkg.in: go get http://gopkg.in/blang/semver.v1