Chao equations

This PR fixes issue #66 . The major changes are
* All Chao models use small-sample correction
* `estimateR` estimates the variance of Chao1 using the bias-corrected equation. However, the equation is based on Chiu et al., Biometrics 70 instead of the EstimateS web page. Changing the variance calculation was the original issue.
* `specpool`  uses basic Chao (like earlier), but now switches to bias-corrected one if there are no doubletons (this follows EstimateS).
* All documentation is updated to these changes.