Switch to the new import paths for go.tools

Clearly this has API implications: the context object is changing
type throughout. Code that uses this library will also need to be
updated to the new import paths.

https://groups.google.com/d/msg/golang-nuts/eD8dh3T9yyA/l5Ail-xfMiAJ
Merge pull request #10 from pteichman/tools

Switch to the new import paths for go.tools