added support for PostgreSQL schema builder 🎉👌

- added a new config. option dont_drop, allows to define table that
   will be excluded during dropAllTables(), has a default value
   spatial_ref_sys for PostGIS
 - implemented fluent commands, used only by the PostgreSQL Comment
   command
 - added tests for PostgreSQL schema builder
 - reworked tst_Migrate to be able to run on all supported connections,
   currently PostgreSQL and MySQL
 - updated docs

Unrelated:

 - added a new reference class IndexDefinitionReference for a nice API
   during index definition, contains algorithm and language
 - unified selectFromWriteConnection() in schema builders

include/include.pri

@@ -73,6 +73,7 @@ headersList += \
     $$PWD/orm/schema/grammars/postgresschemagrammar.hpp \
     $$PWD/orm/schema/grammars/schemagrammar.hpp \
     $$PWD/orm/schema/grammars/sqliteschemagrammar.hpp \
+    $$PWD/orm/schema/indexdefinitionreference.hpp \
     $$PWD/orm/schema/mysqlschemabuilder.hpp \
     $$PWD/orm/schema/postgresschemabuilder.hpp \
     $$PWD/orm/schema/schemabuilder.hpp \