Backport Aggregator changes to 0.7 branch

Passes mima:

[tw-mbp-oscar algebird]$ ./sbt mima-report-binary-issues
[info] Loading global plugins from /Users/oscar/.sbt/0.13/plugins
[info] Loading project definition from /Users/oscar/workspace/algebird/project
[info] Set current project to algebird (in build file:/Users/oscar/workspace/algebird/)
[info] algebird: previous-artifact not set, not analyzing binary compatibility
[info] algebird: found 0 potential binary incompatibilities
[info] Resolving org.hamcrest#hamcrest-core;1.3 ...
[info] Compiling 1 Scala source to /Users/oscar/workspace/algebird/algebird-bijection/target/scala-2.9.3/classes...
[info] Compiling 4 Scala sources to /Users/oscar/workspace/algebird/algebird-test/target/scala-2.9.3/classes...
[info] Resolving com.twitter#util-core_2.9.2;6.3.0 ...
[info] Compiling 4 Scala sources to /Users/oscar/workspace/algebird/algebird-util/target/scala-2.9.3/classes...
[info] algebird-core: found 0 potential binary incompatibilities
[info] algebird-bijection: found 0 potential binary incompatibilities
[info] algebird-util: found 0 potential binary incompatibilities
[info] algebird-test: found 0 potential binary incompatibilities
[success] Total time: 18 s, completed Dec 4, 2014 9:40:45 AM