Add prominent links to the API docs in README

Section headings in the README now link to the API documentation on godoc.org
Add InDelta() for comparing floats.
Add InEpsilon().
Refactor InEpsilon() to use InDelta().
Add InDelta() and InEpsilon() to docs and remove doc duplication.
Revert "Add InDelta() and InEpsilon() to docs and remove doc duplication."

This reverts commit d8398675f90a9d8c93784ecf08cb4edc78bcc99d.
Add to forward_assertions and fix docs.
Merge pull request #66 from bhenderson/master

Implement InDelta() and InEpsilon()
Merge pull request #65 from obeattie/master

Add prominent links to the API docs in README
Serialize calls to Mock.Called.