start adding '.. doctest::' directives to manual
some doc/manual fixes found by doctest
Merge pull request #4897 from JuliaLang/mn/doctest

close #52 (run doc example code as a test suite)
remove trailing newline from show(::SparseMatrixCSC)
Disallow the fish shell; default to /bin/sh if not overriden.

Also Add in a warning notice when auto-changing the user's shell