support t-tests

This pull request adds t-test support.
Three kinds of t-tests are included:

* Student's t-test (assume variance equality)
* Welch's t-test (not assume variance equality)
* paired-sample t-test (samples are paired)

All of them are supported in R and unit tests are checked with reference to it.

I know that @Shimuuar is working on a new API of hypothesis testing (#25). If you think this pull request is too early because the next API will come out soon, I'll wait for that and retract this PR for a while.