clean up the implementations of generic methods for truncated distributions
reimplemented mean, var, entropy for truncated normal
introduce TruncatedNormal to directly construct a truncated normal distribution
incorporate the testing of truncated normal to the standard testing framework for continuous distributions
fix sampling for truncated normal
Merge pull request #295 from JuliaStats/dh/trunc2

Reimplement Truncated and Truncated{Normal}. Fix #292