Allow using Base.show_default instead of custom show

[lbenet]

This allows to use the default function in Base for show instead of the customized one. This is important, e.g., when saving series to a file to load them later.

[coveralls]

Coverage decreased (-0.02%) to 98.066% when pulling d6f53d5 on lb/base_show into 3b0fd33 on master.

[lbenet]

Tests in Julia 0.7 are not passing because they rely in IntervalArithmetics v0.13.0. Once a new version of is tagged, tests will pass (they pass locally using current master). I propose to merge this, and wait for the necessary tags to release a new version.

@dpsanders Do you have any comments?