postgres feature branch slipped into the generated documentation
relicense from AGPL to MPL

there was some criticism regarding the choice of AGPL3. a valid point
is that the generated code is probably licensed AGPL3 too, due to
templaing being used.

licensing this MPL 2.0 should fix this problem, only the generated
database access code has to be licensed MPL 2.0, the rest of the program
can have it's own license (see
https://www.mozilla.org/en-US/MPL/2.0/FAQ/ question 11).
fix sqlite tests

i should really wait with tagging versions until the tests are done :)
fix wrong foreign constraint names in sqlite

every foreign constraint but the first of each table had a wrong name
generated. the name was generated as parent-table-name + constraint-id
instead of table-name + constraint-id (as done for the first foreign
constraint of every table). this led to duplicated names in the
generated go code.
add note about using SQLite features like fts5
add clone method, update and fix docs
make sqlite type maps a parameter to sqlite.DB
add type map to sqlite

the driver functionality of sqlite just doesn't really cut it.
the type map implementation still tries to match the type affinities
and storage classes as described in [1], while giving some flexibility
regarding which types are used in the generated code.

[1] https://sqlite.org/datatype3.html
fix broken INSERT code generation
hopefully unique foreign constraint names with sqlite
guard against empty/nil values in output
unexport sql query consts
update foreign docs
fix appending empty foreign constraint
fix several bugs in foreign constraint handling

reworked the template for foreign constraints, they now don't rely on
other generated methods anymore.
fix bugs in mysql foreign constraint reading.
fix sqlite wrongly naming first foreign constraint
add missing file for docs