DOC: Add PPA install instructions for Ubuntu
DOC: Improve documentation index
DOC: add more details about release steps
CI: Add script to help download wheels
Remove _lda.c from repository, should be generated using cython

Documentation adjusted to reflect this.
TST: Add tests covering case when user provides sparse matrix
Support sparse document-term matrices
DOC: Mention lda-users mailing list
Merge branch 'feature/sparse-tests' into develop
CI: Generate .c files using Cython in CI setups
PEP8
CI: Fix appveyor, fetch Windows scipy wheels needed for tests
DOC: update release instructions to reflect change in c code generation