Initial self-hosting for tests.

Initial self-hosting for tests.

* pom.xml: Change the scope for log4j, to hopefully make it
available to the test code which uses it indirectly via jOOQ.
* src/main/java/org/glom/web/server/ConfiguredDocument.java: Allow
self-hosting, though we only use it for testing.

* src/main/java/org/glom/web/server/libglom/Document.java:
example rows: Use a map instead of a list for each row of values,
so we know what field they are for, instead of relying on the sequence
being correct. This is not very efficient, but it does not really need
to be.
* src/test/java/org/glom/web/server/libglom/DocumentTest.java
testReadTableExampleRows(): Adapted.

* src/main/java/org/glom/web/shared/DataItem.java: Added getValue()
that returns an Object, for generic use. Note that Object seems to be
the implicit base even of double.
* src/main/java/org/glom/web/shared/libglom/Field.java: Add getSqlType(),
for use in CREATE TABLE SQL queries.
* src/test/java/org/glom/web/server/SelfHoster.java: Add this class
to do self-hosting of PostgreSQL databases via its command-line
utilities, based on Glom's C++ code in test_selfhosting_utils.cc and
backends/postgres_self.cc. This is incomplete - it needs more
warnings about failures and it needs to clean up properly when things
fail.
* src/test/java/org/glom/web/server/SelfHostExampleTest.java: A simple
test of this new class.