Allow not linking to datasets when importing records

With the -j flag, harvest objects are not linked to datasets when
importing. This is useful sometimes when importing records for the first
time.
Allow defining segments of harvest objects to import

Useful when importing large number of objects, as it allows
parallelization
Do not use repo.are_tables_created

When checking whether the core tables have been alredy created  it is
best to use package_table.exists(), as are_tables_created reflects the
tables, causing conflicts with other extensions.

This allows ckanext-harvest and ckanext-spatial to be used together on
ckan 1.8 onwards.
Do not store the object when importing
Allow showing harvest objects by default (on the default auth profile)
Fix the harvest object show call
Strip spaces from url when creating or updating a source
[#2] Update README link
Merge branch 'master' into dgu