Firebird backend for Django 1.6.x is updated

Here are a few fixes for v1.6.x Release Candidate 1 commited into master
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Fixed missing date_interval_sql implementation. #21
Fixed datetime_trunc_sql NotImplementedError. #20
Fixed missing SQLDateTimeCompiler. #19
Fixed BigIntegerField is not supported in introspection. #18
Fixed BinaryField is not supported in introspection. #17
Fixed missing method name. #11
Fixed setup.py due to README extension change. #12
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Also Django Firebird stable 1.5.2 is uploaded to pypi with a few fixes backported from master branch (if you use Django 1.5.x)