Add pagination `padding` option to #search.

This allows you to pad a number of records that is not a multiple of the page size.
alias num_pages to total_pages

for compatibility with kaminari's paginate helper
Do not throw errors when index becomes out of sync with database - for #31
Merge branch 'master' of https://github.com/ankane/searchkick
Merge pull request #198 from kieran/master

alias num_pages to total_pages
Added custom exception types for #201
Added InvalidQueryError and test for unsupported version
Updated changelog
Subclass Elasticsearch::Transport::Transport::Errors::BadRequest for backward compatibility
Added custom message for bad mapping for #199
Determine id consistently
Removed hack
Added note about route
Fixed offset_value - closes #206
Version bump to 0.7.5
Merged padding option
Merge branch 'jtomaszewski-feature/pagination_padding'
Added facet options for facet stats
Added simple test
Added section about facets stats in README