Allow mapping to have an empty string as name

The mapping name check in addMapping was previously for truthiness which
meant that an empty string would evaluate to false. This causes an error
in the _serializeMappings function when a mapping has an empty string
for its name because that check is for != null and thus an entry is
expected in _names and it does not exist.
Add test to verify empty mapping name doesn't fail
Allow mapping to have an empty string as source

addMapping was checking source for truthiness instead of just null /
undefined. Everywhere else just checks for null / undefined which could
lead to an error.
Merge pull request #121: Allow mapping to have an empty string as name.

Fixes #121.
Bump to 0.1.36