parser: Add series parsing

parser: Add series parsing

It is now possible to parse and store series, so do just that.
The parsing at the moment is based on both RFC822 headers and
subject lines.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Andy Doan <andy.doan@linaro.org>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Tested-by: Russell Currey <ruscur@russell.cc>