fix hook_requirements()

from hook_requirements api documentation:

Note that this hook, like all others dealing with installation and updates, must reside in a module_name.install file, or it will not properly abort the installation of the module if a critical requirement is missing.

we need to:

- move it to install file
- use get_t() for translations
- show a more descriptive message when xapian bindings are not installed, by adding a "you need to install xapian bindings" in addition to the real include error

commited