requirements.txt: list requirements for pip
requirements.txt: add sqlalchemy & feedparser

from .travis.yml && travis.yml: use requirements.txt.
INSTALL.md & requirements.txt: remove pysqlite.
INSTALL.md & requirements.txt: remove Twisted.
INSTALL.md: mention pip install -r requirements.txt
Merge pull request #600 from Mkaysi/requirements

Travis: install pip packages with requirements.txt