risch: factor out common expression.
risch: Fix WrongResult for x*exp(-k*x).

Fix issue 3886: Incorrect Piecewise result from risch_integrate
http://code.google.com/p/sympy/issues/detail?id=3886
Merge pull request #2170 from jrioux/integrals

Fix WrongResult for risch_integrate(x*exp(-k*x), x)
Add Ramana Venkata to the AUTHORS/aboutus. Welcome to SymPy!
Following are the changes and enhancement proposed in this PR.

1. added a new method nnz in sparse, which returns the number of non-zero elements of Matrix.

2. added a function doktocsr to convert dictionary format to csr format. This format will be used for sparse arithmetic in the next PR. I was not sure where to place them so I created a new file mat.py.

3. added tests
Fixing failing tests, but there is a trailing white space in docstring of __neg__ method of sparse. I am not able to remove it yet, but will do it subsequent commit.
Addressed comments. I left initalizing of  A, IA and JA to empty lists, as I think they serve as good documentation.
Fixed bugs in the current implementation. Also added some new tests
Fixed the test failure, it was failing because it was not handling the case of multiple zero rows at the end.
DOK to CSR
Removed mistakengly added blog feed
Addressed comments. Wrapped explanations in docstrings
A function to convert dok to an orginal matrix
Added a new function to convert CSR format to a 'user-facing' type matrix, SparseMatrx. Also removed sympy doc files that were mistakengly added last time