Add numeric computation doc page

Discusses lambdify, ufuncify, and Theano options for numeric evaluation
add link to numeric computation in modules index
replace x.length with n
add evalf
Blacklist numeric-computation.rst tests if numpy or theano modules are not imported
Fix tests in numeric-computation.rst
Install numpy for numeric-computation.rst if TEST_THEANO is true
Merge pull request #13 from skirpichev/mrocklin-computation-docs

fix travis failures
Fix sphinx test in travis: drop evalf-label
Install scipy in travis (TEST_THEANO) for numeric-computation.rst
Fix trailing whitespaces and reformat long lines
Merge pull request #14 from skirpichev/mrocklin-computation-docs

More fixes
Install scipy deps, including blas, gfortran, etc
Merge pull request #15 from skirpichev/mrocklin-computation-docs

Try to install scipy deps, including blas, gfortran, etc
semicolons!
Merge pull request #16 from skirpichev/mrocklin-computation-docs

fix travis failures
Merge pull request #2400 from mrocklin/computation-docs

Numeric computation doc page