fix regression confidence interval, clean up confidence interval api

[erikbern]

it was kind of poor for various reasons

1. was using the nonstandard 5-95% rather than 2.5-97.5%
2. confidence_interval is really long, let's just do ci instead
3. the confidence intervals for the regression model was wrong, i was confusing variance with standard deviation

[coveralls]

Coverage increased (+0.8%) to 68.798% when pulling b40c25e on confidence-intervals into 896b02c on master.

[coveralls]

Coverage increased (+0.8%) to 68.798% when pulling b40c25e on confidence-intervals into 896b02c on master.