RUBY-615 adding license/copyright source headers

As recommended by ASF policies for the Apache 2.0 license, I've added back the source headers to all "non-trivial" source files in the project.

Things I skipped:

* The benchmark suite (I'm going to delete this)
* All trivial/boiler-plate files
* 3rd-party files (Removed in commit from RUBY-618)
RUBY-615 adding required notice file

Adding the required NOTICE file for compliance with our Apache 2.0 license. Since it's not required to include non-bundled dependencies, this file only contains our own copyright information.

With the changes I've made in RUBY-618, the only bundled dependency we have in this project now is with the BSON package from the Java driver. However, since it's also our own and licensed under Apache 2.0 we aren't required to list it here.
RUBY-615 removing unused benchmark suite

I think we can safely remove these at this point. AFAIK we're not really using them anymore and these performance tests are covered in a separate project now that looks for performance regressions on all drivers (mongo-performance).

This isn't directly related to RUBY-615. I'm including it here because I'm deleting these files rather than adding the license and copyright source headers.

In additional to cleanup and license compliance, getting rid of this unused benchmarking suite will allow us to filter out the large pdf file here and reduce the size of our repo quite a bit.