Main process no longer calls init_node.

This fixes bugs related to calling init_node multiple times in the same
process. Main process now has its own signal handler for shutting down
cleanly.
Remove dependency on pymongo > 2.3

In older versions of pymongo, Connection serves the same purpose as
MongoClient. Updated scripts to use Connection instead of MongoClient.

This allows the package to work with the existing rosdep definitions for
python-pymongo (shich use the .deb version).
Cleaned up boilerplate in mongodb_log package.xml

Removed a bunch of XML comments (that came from the package.xml
template) from the package.xml file. Added pymongo as a run dependency.
Remove dependency on bson > 2.3

Use old hook/default interface to avoid having to install bson 2.3 from
pip.
Dynamically choose MongoDB API

Use Connection if using an older mongopy, otherwise use MongoClient.
Merge pull request #61 from clearpathrobotics/dependency-fixes

Remove dependency on pymongo 2.3+