Add offset query parameter.
Add paging feature to the EnginioModel.

Paging allows to load and present data partially page by page, which
should have quite big impact on an application performance. The feature
is off by default as it potentially can expose race conditions. Docs
are still missing.
Unify most common strings literals.

It reduces size of library by 20k and potentially may improve data
lookup performance.
Merge pull request #33 from nierob/paging

Merge feature branch for paging and unify common string literals.