BUG: fix moments of R distribution
BUG: fix moments of burr/fisk distributions
BUG: fix a typo in F distribution skew
BUG: make F stats not raise a division-by-zero warning.

There is a generic problem with broadcasting:
moment and stats methods do not properly broadcast. In this
commit, a failing case (where dfd and dfn have different
length) is marked as a knownfail.
MAINT: a trivial grammar fix in a comment
BUG: fix the formulas for the 3rd and 4th moments of fatiguelife distribution.

Also perform the pdf calculations in log space to improve
accuracy.
BUG: fix the moments of nct distribution
BUG: fix formulas for foldnorm moments
TST: avoid test failures on numpy < 1.7
Merge pull request #2879 from EvgeniBurovski/burr

[stats] fix formulas for higher moments of R, Burr, Fisk and F distributions