created table layer_namespace to store the many to many unique mapping of layers and namespaces
changed v1 api to provide a list of namespaces for each layer
changed namespace detector to use all registered detectors to detect namespaces
updated tests for multiple namespaces
Fixes#150
This removes the `types` package instead moving the contents to the
top-level clair package.
This change also renames the `Priority` type to `Severity` in order to
reduce confusion.
This change also removes the IsValid method and replaces it with a safe
constructor to avoid the creation of invalid values.
Many docstrings were tweaked in the making of this commit.
