Fix NoMethodError for Zestimateable when valueChange doesn't have a duration attribute

Sometimes <valueChange> inside <rentzestimate> doesn't have a duration
attribute.  This caused a NoMethodError: undefined method 'value' for
nil:NilClass error.
Bump patch-level version to 0.0.6
Add xpath_if_present helper method to use when data might not be present
Merge pull request #21 from danielberkompas/fix_zestimateable

Fix common scenarios where xml nodes are not present
Document 0.0.6 changes
Release 0.0.6