Created mailer factory to better separate VuFind-specific configuration issues from actual mailer functionality.

Started loading configurations through a custom plugin manager instead of static \VuFind\Config\Reader.

Fixed resolver drivers to use HTTP client instead of file_get_contents; this adds proxy support and improves testability.

Bugfix: Only convert month part from string to number if not already provided as number.
Error occures when input date has this format: 31/12/2013
Additional cleanup: Replaced deprecated split() with explode()

Made XSLT importer custom class handling more flexible (and made the nasty class truncation logic optional, so we can start to move away from it).  Updated comments in configuration files to match.

Created Composer configuration and rearranged vendor directory to match; this should make it easier for people to add additional dependencies (VUFIND-740).

the original code no longer seems to be maintained, so it is not composer-friendly.

Related to VUFIND-504: truncate the ID list when doing a tag search to avoid MaxBooleanClauses error.