Wrappers for ntheory
Implement `Constant` class to represent symbolic constants in CSymPy
Replace `Symbol pi` by `Constant pi`
Fix a typo
pip: remove deprecated "--use-mirrors"
Travis: don't compile Cython

Cython is used in a pure Python mode, which speeds up the compilation and also
fixes #308.
.travis: remove a typo in the script
.travis.yml: Combine the two 'if' statements
Merge pull request #312 from thilinarmtb/constants

Represent constants in CSymPy
Merge pull request #313 from certik/travis_cython_fix

Travis Cython Fix
Tests for sorting of integer vectors in primitive/nth_root_list
Merge pull request #302 from isuruf/ntheory_wrappers

Wrappers for ntheory
Merge branch 'master' into arb_II
Revert the changes to `visit(Symbol)`
Implement visit for Constant
Remove redundant semicolons