Change type of the API version to integer

The CKAN logic uses integers when dealing with the API version, e.g.
making checks which API version is in use. Currently, the harvester
uses strings to identify the API version. Instead of dealing with
type conversion the harvester could use integers directly.

This commit fixes okfn/ckanext-harvest#36. When the API version is
parsed from the configuration it is passed through the int() function.
This way the harvesting will still work even if a harvest source was
configured with a string API version which makes this commit backward
compatible.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Add exception handling for the API version parsing

I have added try-except clauses in order to prevent the process from
crashing if a non-parsable integer is used for the api_version option.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
Fix api_version check in the group importer code

I have forgotten to update one check for the api_version 1 in the code
responsible for the remote group import feature. This commit fixes that.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>
[#36] Move validation code to validate_config

This ensures it is checked whenever the source is edited or created.