* requirements.txt: removed MySQL and SQLAlchemy
* query_builder.py: enable hl.q, wrap search with relevance wrapper if defined
* forms.py: database selection is now set in a hidden variable
* search_macros.html: database selection now hidden from optional search menu
* config.py: added relevance, highlighter wrappers, modified defaults
* requirements.txt: oops, put back dependency on MySQL
query_builder.py: reworked code that detects sort option and enables relevance ranking
* config.py: use more flexible SOLR_RELEVANCE_FUNCTION rather than *_WRAPPER
* results.html: fix bug with extra semicolons at end of author list