Some fixes to the magic import system

- Stop adding the directory of every .capnp file to the import path. If a .capnp
  file wants to import a file in its own directory, it should use a relative
  import. Fixes #278
- Stop using /usr/include/capnp as an import path. This is incorrect. It should
  only be /usr/include.
- Stop allowing additional paths to be specified for magic imports. This leads
  to inconsistencies. More specifically, the way that a nested import like
  `ma.mb.mc_capnp` gets imported by python, is to first import `ma`, then import
  `ma.mb`, and finally `ma.mb.mc_capnp`. Pycapnp's magic importing is only
  involved in the last step. So any additional paths specified don't work for
  nested imports. It is very confusing to only have this for non-nested imports.
  Users with folder layouts that don't follow pythons import paths can still use
  `capnp.load(.., .., imports=[blah])`.

test/schemas/child.capnp

@@ -0,0 +1,5 @@
+@0x9afc0f7513269df3;
+
+struct Child {
+  name @0 :Text;
+}