Merge pull request #6 from embaldridge/master

All looks good to me. Merged.
Tentative fix for the precision issue
Fix bug in pln.cdf for multiple input values
Adopt transformation in pln_gen for extreme cases.
Add function to obtain MLE parameter of upper-truncated geometric
More stable algorithm to obtain MLE parameter of Yule distribution
More stable algorithm to obtain MLE parameter of the genrealized Yule distribution

This new algorithm is based on the idea from Garcia 2011 (Applied
Mathematics and Computation). It has NOT been extensively tested;
however it appears to return the same results as the old algorithm when
it does converge while having better converging properties than the
latter.
Modify trunc_geom_solver to circumvent the precision problem when upper_bound is high
Modify the searching bound in trunc_geom_solver again for speed