Added a few lines to docs
simpler real solution of ``roots_cubic`` in some case
Fix the docstring of Matrix.det

The method name for the LU method should be "det_LU", not
"lu_decomposition". We should perhaps change the code to match the docstring
instead, but that would break compatibility.
used ``_mexpand``
eval_determinant for Identity and ZeroMatrix
used ``_mexpand``; small change in ``check_solutions``
Altered docs a bit
Merge pull request #2470 from pernici/cubic1

simpler real solution of ``roots_cubic`` in some case
Merge pull request #2472 from pernici/dioph

used ``_mexpand``
Merge pull request #2473 from mrocklin/eval-determinant

eval_determinant for Identity and ZeroMatrix
Minor changes
Merge pull request #2466 from Manoj-Kumar-S/docs

Added a few lines to docs
Another fix in the docstring of Matrix.det
Merge pull request #2471 from asmeurer/matrix_det_lu

Fix the docstring of Matrix.det
Merge branch 'master' of git://github.com/sympy/sympy