Fix init_session(ipython=False) for non-unicode terminals.

InteractiveConsole is another case were we should not force
use_unicode=True. Instead, let's rely on the code that checks
sys.stdout.encoding to automatically set this to the correct
value.
init_printing: Update a comment.
issue 3434: warnings from matplotlib when starting isympy
the issue was that matplotlib.pyplot was being imported before
pylab was initialized. initializing pylab after the import of
matplotlib.pyplot caused a warning to be printed to the console.
some plotting methods don't work in py3k without this. objects are making
their way into sympy's cache.
hack to get the pyplot fix to work with matplotlib/py3k
unit tests to ensure that interval objects in the plotting module are hashable
this came up in issue 3434
add comment and tests, remove/reformat some old comments
amend test for case where matplotlib is not installed
clarify regression test
A new utility for integrals
Added support for summations in finit diff
Removed integral change
finite diff function that works for multiple indexes
Wrapped the docstrings
Print unnamed Dummy symbols more prominently

Previously, they were just printed as _12 or pretty printed as just 12, which
looked like a number in an expression.  This way, if a Dummy leaks out, it is
clear.
Merge pull request #2127 from SaurabhJha/kauers

Kauers
Made finite_diff efficient
Fix issue 3700

- Manualintegrate changes:
    - Rewrite rule: do not use if integrand cannot be evaluated with manualintegrate
    - Substitution rule: do not use ConstantTimesRule if constant == 1
    - Added test
Fix typos in usage of manualintegrate in _eval_integral
integrate: do not use manualintegrate if the result has dummy variables