This puts config in its relevant location and moves functions around
loading config files into the main package.
As a side effect of removing cyclic imports for the API config, the
context library is no longer used.
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.
