Swap BaseURL for baseURL throughout.

Running the tests produces the following error:

$ go test
# gographite
./gographite.go:42: unknown Client field 'BaseURL' in struct literal
./gographite.go:47: c.BaseURL undefined (type *Client has no field or method BaseURL)
FAIL    gographite [build failed]

Looks like BaseURL was unexported in https://github.com/gorsuch/gographite/commit/9c6aaf2ffa8dd1267b20e44b2a765b8b111977b5 but a few stragglers remain. Now everything's green:

$ go test
PASS
ok      gographite      0.011s
Merge pull request #1 from anachronistic/fix-base-url-error

Swap BaseURL for baseURL throughout.