src/session.scm (session?): New procedure.

* src/session-type.c (guile_ssh_is_session_p): New procedure.
* src/session-type.h (guile_ssh_is_session_p): Export.
* src/session.scm (session?): Export.
* NEWS: Update.
Merge branch 'master' into tests-implementation

Conflicts:
	ChangeLog
tests/session.scm ("session?"): New test case.
tests/session.scm ("comparsion of sessions"): New TC.

* tests/session.scm ("comparsion of sessions"): Use one test case
  instead of two separated TCs (equal?/not equal? test).