Pretty printing of Integral(x, x)**2.

Wrap an Integral with brackets when raised to a power. Do the same
for most objects, excepts if it is an atom (doesn't need brackets)
or a function (which handle printing powers differently already).

Fixes issue 3260.
Remove a couple TODOs.
Merge branch 'master' into printing

Conflicts:
	sympy/printing/pretty/pretty.py
	sympy/printing/pretty/tests/test_pretty.py
Merge pull request #1788 from jrioux/printing

Pretty printing of Integral(x, x)**2.