README.md: add build status

Display Travis build status on README.md.
Merge pull request #30 from adericbourg/patch-1

README.md: add build status
Implement $push 2.6 support with $each, $position, $sort, and $slice.
Rewrite compareDBObjects to match mongo 2.6 behavior.
Fix identification of LazyBSONObject as a DBObject.
Change imports style to match project.
Move compareToNullable(...) utility method to Util.
unncessary cast
start to handle driver 2.12.0
add new aggregate
closes #35
remove deprecated
[maven-release-plugin] prepare for next development iteration
latest version of mongodb drivers
modify README
[maven-release-plugin] prepare release fongo-1.5.0
[maven-release-plugin] prepare for next development iteration
Clarify sort order.
Merge pull request #34 from htmldoug/master-push-2.6

Implement $push 2.6 support with $each, $position, $sort, and $slice.
add contributor