Renaming "Observer" to "Listener" for consistency in HTTP/2

Motivation:

We currently have a mix of "Observer" and "Listener" for interface
names. We should make them all "Listener" to be consistent.

Modifications:

Renamed Http2DataObserver->Http2DataListener and
Http2FrameObserver->Http2FrameListener.

Result:

Listener interface names are consistent.
Exclude bin directory from git Motivation:

The 'bin/' directory is currently not in the .gitignore file.
Eclipse creates this directory to support IDE operations.
These directory is generated and not related to source code and therefore does not belong in git.

Modifications:
-Add 'bin/' to .gitignore

Result:
-No more 'bin/' directory confusion for eclipse users.