Fix nested sum evaluation.

Fix the evaluation of nested sums that contain a KroneckerDelta.
Also fix the evaluation of sums that contain a Piecewise, which
is the return value of deltasummation in most cases.

Fixes issue 3924: Evaluation of nested sums.
http://code.google.com/p/sympy/issues/detail?id=3924
3861: Fix the latex printing of 'array'.
3861: Change the default latex matrix type.

Use 'smallmatrix' for mode='inline', use 'array' otherwise.
Determine the default latex matrix type according to size.
Only have the latest version in one place

From now on, anything that wants to reference the most recent version of SymPy
should import sympy and use sympy.__version__.  Anything else should be
assumed to not update when we release.
Merge branch 'master' into 0.7.3
Fabric: get the version number automatically

Also, build the pdf docs
Improve comments
Fabric: checkout a branch
Allow to specify a branch

This finishes my changes from the previous commit
Enable installing docs packages
Merge pull request #2253 from sympy/0.7.3

0.7.3
Bump version to 0.7.3-git
Merge pull request #2195 from jrioux/printing

3861: Change the default latex matrix type
Merge pull request #2245 from jrioux/concrete

Fix nested sum evaluation.
Improve the atan2 function