Work on EXP-160: Refactor extractors.

Refactor extractors so they can be configured on appropriate lifecycle phases.
EXP-168. Normalize naming of Avro Classes.

The names of Avro records for columns and filters have been changed from
'*Spec' style to 'Avro*' to match other classes and distinguish
from records like 'AvroInputSpec' (which legitimately should end in 'Spec').

issue: https://jira.kiji.org/browse/EXP-168
review: https://review.kiji.org/r/975
Update dependencies for release.
Updated RELEASE_NOTES.txt for release.
[maven-release-plugin] prepare release kiji-express-0.8.0
[maven-release-plugin] prepare for next development iteration
EXP-186. Update Scalding version used by KijiExpress.

issue: https://jira.kiji.org/browse/EXP-186
review: https://review.kiji.org/r/984
EXP-173. Users need to be able to page through columns with many versions.
Users of express may now request that column paging be enabled. If paging
is enabled, a PagedKijiSlice is returned in the associated tuple field. A
PagedKijiSlice allows users to iterate over all cells in a column, while
limiting the number of cells in memory.

Reviewed By: Robert Chu
add test