First work on implementing Solr support.
Solr:
 - refuse to start if no Solr URL has been entered
 - index categories in a simpler way
 - index all subjects/objects, also by predicate.
Solr stuff.
 - Added a simple matcher. Nice for sidebars etc.
 - Implemented most of the Query-model syntax --> to solr mapping

Still to do:
 - Delete doc -> delete from solr
 - Sorting
 - Implemented sorting
 - Implemented delete from solr index
Added readme file.
solr store: add the IsA to the module-callback for extra solr props.
Make hasobject/hassubject etc. required properties for Solr, so search
results are more equal to the non-solr results.
m_category: retrieve a subtree as a flat array with m.category[id].all_flat

solr: retrieve facet info as associative array for template addressing.
Solr - support for dynamic int fields; Solr URL from zotonic config is
now used, so can be used with multicore solrs.
Solr: add connection:close header so the http client does not crash
Fixed typo in sendmail-method of mod_emailer, and used another
sendmail syntax: sendmail -f <from> <to>.
Reverted accidental checkin.
Work in progress.  Now exported module functions pid_observe_xxxx will automatically observe the xxxx event and will also receive the pid() of the module as the first parameter:  pid_observe_xxxx(Pid, Xxxx, Ctx)
action_moreresults: solr faced search results has an 'ids' sublist
that contains the ids. Take this into account in calculation of the results.

Hg: Enter commit message.  Lines beginning with 'HG:' are removed.
Solr: make "match" search more robust.
Let mod_search_solr crash on startup when the connection to solr is refused.
Make sure that the solr search terms are lists.
Added long(er) timeout on solr searches.  Fixed problem where the match gen_server handler  was calling search with an invalid query.
Fixed problem where a solr text search for the empty string gave a Java exception.
When searching multiple categories to solr, make the syntax like +(category:foo category:bar)