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.
Piecewise._eval_interval for cond. indep. of integration var.

This fixes the Integral of a Piecewise when the conditions are
independent of the integration variable.

Before:

>>> integrate(Piecewise((0, Eq(y, 0)), (x*y/sqrt(y**2), True)), (x, 1, 3))
4*y/sqrt(y**2)

After:

>>> integrate(Piecewise((0, Eq(y, 0)), (x*y/sqrt(y**2), True)), (x, 1, 3))
Piecewise((0, y == 0), (4*y/sqrt(y**2), True))
Fix issue 3649

The problem was that Integral._eval_subs was returning an instance of
Integral, so that NonElementaryIntegral.subs() was returning an instance of
Integral, and hence failing an isinstance check. I changed all methods of
Integral to use self.func(*args) instead of Integral(*args).  But we need to
audit all of SymPy and make sure we are doing this everywhere.  See issue
3652.
Fix tests in Python 3.3 with --no-subprocess

See issue 3629.  Test by running ./bin/test --no-subprocess in Python 3.3.
Update wiki references (point to githib/wiki)
Merge branch 'master' into old-wiki-refs
Merge branch 'master' into remove-old-wiki-refs
Drop references to
https://github.com/sympy/sympy/wiki/old-wiki-Sympy-Development
Drop obsoleted refs to
https://github.com/sympy/sympy/wiki/old-wiki-Discussion
use subprocess module in preview.py instead of os.system
make preview function threadsafe again
added support for a few new kwargs to the preview function + cleanup
use more kwargs for SymPyDeprecationWarning
updated docstring
reworded docstring
fix test failures
removed call to sleep()
fix doctest error

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
preview: cleanup