Update the latest Groonga version
Support reference weight vector

It requires Groogna 4.0.4 or later.
README: follow required Groonga version.

Reference weight vector is supported Groonga 4.0.4 or later.
When cross compiling for Windows, it should not check host pkgconfig result
add clean:groonga task
Use ranguba.org as homepage
memory-pool: support accessor

It will reduce memory usage without GC!
memory-pool: support snippet

It will reduce memory usage without GC!
Fix indent
Use "Groonga" form
memory-pool: support variable

It will reduce memory usage without GC!
memory-pool: revert adding variable to memory pool

Because it is closed by expression. We don't need to close it.
Add missing have_finalizer check
Split running finalizer phase and unbind phase

Because some objects don't have finalizer but they should be unbound
related Groonga objects to detect whether the Groonga object is closed.
memory-pool: support expression

It will reduce memory usage without GC!
Support closing accessor by GC
Fix a crash bug by GC

It fixes a GC related crash when GC is run while Table#group.
Avoid double unbind
accessor: use floating object mechanism

It fixes a crash bug that is introduced at
5976731352ed992ed3b6c4c0434e9ffccc03477c.

Reported by cosmo0920. Thanks!!!
test: add test name to database path for easy to debug