agent: Bundle bindings as a shared library

Instead of creating an agent executable, bundle the bindinds into a shared
library.  That way, we're adding an extension that Guile can load at runtime.
This shared library can be installed along with libjami so that users can use
the full power of Scheme to control their Jami accounts.

Also create a `guile-wraper' that compiles against the configuration flags.
This is necessary when building libjami with ASAN because then the executable
either has to LD_PRELOAD ASAN or compile against it.  This minimal wrapper
really just boot guile and start a REPL.

Change-Id: I984d211ff03322b102ebe8650950ac8487711e9d