feed enough data into ks and adf tests

[mabrek]

fixes #43

[astanway]

just wondering - what's the basis for 20?

[mabrek]

20 comes from default 'startup' value of detectChangePoint function from http://cran.r-project.org/web/packages/cpm/cpm.pdf
The idea is that kolmogorov-smirnov test is too noisy when sample size is small.
Another reason is that adf test uses autocorrelation with maximum lag 10 so the sample size should be larger than 10.