Small typo fix.
Adds exception chapter for the first time.
Merge pull request #18 from ThyArmageddon/master

Merging introduction changes to the modules chapter.
Merge branch 'master' of github.com:kushaldas/pym
finally statement in exception chapter.
Fixes a few typos
Merge pull request #19 from Sanjiban/master

Fixes a few minor typos
Fixed some typing mistake
Merge pull request #20 from iamsudip/env

Fixed some typing mistake
Typo fixes for virtualenv.rst