Introduce Regexp-matching predications with SexyScopes::Arel::Nodes::RegexpMatches

TODO: we now need RDBMS-dependent specs plus specs for methods added to visitors
Merge branch 'master' into regexp
Add lib/arel/visitors_extensions.rb containing (monkey-)patches for Arel::Visitors
Regexp support: added Arel 4.0 compatibility
Add gemfiles/*.lock to .gitignore
Add documentation for Regular Expression support
Merge branch 'regexp'