tox: Automatically test with all FS, ZEO and NEO backends

/cc @jm

tox.ini

 # wendelin.core | tox setup
 [tox]
-envlist = py27-{ZODB3,ZODB4}-{numpy18,numpy19}, py34-ZODB4-{numpy18,numpy19}
+envlist = py27-ZODB3-{fs,zeo,neo}-{numpy18,numpy19}, {py27,py34}-ZODB4-{fs,zeo}-{numpy18,numpy19}
 # (NOTE ZODB3 does not work on python3)
+# (NOTE NEO does not work on ZODB4)
 
 [testenv]
 deps    =
@@ -14,9 +15,21 @@ deps    =
     # latest current ZODB _4_
     ZODB4:  ZODB3 >=3.11
 
+    neo:    neoppod >= 1.3
+    # neo.tests also imports MySQLdb unconditionally
+    neo:    MySQL-python
+
+
+    # zeo: ZEO is already required by ZODB, but for testing utilities from ZEO we also need:
+    zeo:    zope.testing
+
     numpy18: numpy >=1.8.2, <1.9.0
     numpy19: numpy >=1.9.2, <1.10.0
 
+setenv =
+    fs:  WENDELIN_CORE_TEST_DB=<fs>
+    zeo: WENDELIN_CORE_TEST_DB=<zeo>
+    neo: WENDELIN_CORE_TEST_DB=<neo>
 
 commands= {envpython} setup.py test
 # XXX setenv = TMPDIR = ... ?  (so that /tmp is not on tmpfs and we don't run out of memory on bench)