BUG: Fix mean, var, std methods for object arrays.

This takes care to preserve the object type for scalar returns and
fixes the error that resulted when the scalar did not have a dtype
attribute.

Closes #4063.
MAINT: Remove checks for presence of decimal module.

The decimal module has been available since Python 2.4 and we no
longer support Python earlier than 2.6.
Merge pull request #4276 from charris/fix-stat-methods

BUG: Fix mean, var, std methods for object arrays.