Add `.gitignore`.
Rename extension "jansson-json".
Move tests around to support `make installcheck`.
Move things around PGXN-style.

* Move `.c` file to `src/`
* Move `jansson-json--0.0.2.sql` to `sql/jansson-json.sql`.
* Update `Makefile` to generate `sql/jansson-json--0.0.2.sql` on 9.1 but not earlier.
* Update `Makefile` to find stuff itself, rather than know explicitly where to look for stuff.
* Remove the deprecated use of `USE_PGXS` from `Makefile`.

Somehow all warnings are gone now. All tests pass in `make installcheck`.
Point to new file location in META.json.
Restore verbosity in the test output.

But eliminate the NOTICEs. They're eliminated from the main migration script, so should be quiet during installation, as well.
Move documentation around.
Add ability to migrate from unpackaged.
Changes.
Add prereqs, tags, and bug tracker to META.json.

Just guessing that 8.4 is the minimum required PostgreSQL; probably close to right, though needs testing.

Also fixed the GitHub URLs.
Merge pull request #1 from theory/pgxn

Lots of stuff to make it more PGXN-ready