Use the protobuf jar file from the system.

Either sysjarsdir or jars.protobuf can be overridden in local.properties.
Add support for a basic Travis continuous build.

Their environment is based on Ubuntu Precise, missing a number of necessary
packages, so this is only the subset that builds right off (Java and the
"pure Go").
Improve the Go binding for merkletree.
Run the Python tests on our continuous build.
Remove warnings from several Java source files.
Remove a FIXME comment that no longer applies.

This comment was referring to curlpp, which we no longer use.
Avoid the dependency on the makefile from Go.

This avoids the extra step of "make golibs", and also leaves Go in control of
compilation flags.
Minor cleanups in the proto files.