Enhance Instrumentor.class of dtest to be more flexible

Hi,

it would be nice if dtest (from Byteman's contrib) can:

1. injectOnCall with specified condition instead of whenTrue() at
Instrumentor.class
2. installScript to be a public method to allow insertion of any
specific script remotely

I am using it for JBossTS tooling tests and need more flexible way to submit byteman scripts to the remote server.

Please take a look at the proposed patch, it is really short:
https://github.com/istudens/byteman/compare/dtest_patch

I will create a pull request for it in a second.