Implement very basic factorization functionality
Update .travis.yml
Update .travis.yml
Merge branch 'master' into integer_factorization_with_ecm
Update travis to install dependency `libecm0`
Include `ecm.h`
Remove unnecessary `ecm.h`
Fix a typo
Add the missing FindECM.cmake
CMake: Make ECM optional
Fix a typo in FindECM.cmake

In the main CMakeLists.txt we use ECM_LIBRARY, so we need to use it here as
well.
Travis: sufficient to install libecm-dev only
Define HAVE_CSYMPY_ECM
Only include ecm.h if ECM is installed
ntheory: only use ECM if it is installed
Start implementing a slow factorization algorithm
Travis: only keep the "yes" parts

This simplifies a lot how the script as well as results at Travis-CI look like,
but functionality is the same.
Move the ecm.h include from integer.h to ntheory.h

The ECM library is a specialized library that should only be used and included
from the place where it is needed, not the general integer.h file.
Implement factoring by trial division
add _factor_trial_division to ntheory.h