Fix bug related to new `basestring` type in cython 0.20

In particular:
`redshift/sentence.pyx:251:20: Cannot convert 'basestring' object to bytes
implicitly. This is not portable.`
Merge pull request #7 from amacias/develop

Fix bug related to new `basestring` type in cython 0.20
* Integrate merge from pull request about base string in cython0.2