Adds support for configuration of shells from scrapy.cfg
and SCRAPY_PYTHON_SHELL.
config snippet:
cat <<EOF >> ~/.scrapy.cfg
[settings]
# shell can be one of ipython, bpython or python;
# to be tried as the interactive python console
# (in above order, unless set here).
shell = python
EOF
(closes#270, #1100, #1301)
