Don't create markup when source column is nil or empty string.

Destroy markup when source column is set to nil or empty string.

Add pry, autotest
Version bump to 0.2.17
Merge branch 'master' of https://github.com/zetetic/acts_as_wiki

Conflicts:
	VERSION
Don't create markup when column value is missing

Remove markup when column value is unset

Allow subclasses to disable wiki markup

Update RSpec and Bundler
Version bump to 0.2.18
Whitelist attributes in wiki_markups
Version bump to 0.2.19

Make Jeweler work again