Support building against Python 3 (#17)

Add python_version argument to python_configure().

python_configure.bzl

diff --git a/python_configure.bzl b/python_configure.bzl
index 795fa71..8f3eb5e 100644
--- a/python_configure.bzl
+++ b/python_configure.bzl
@@ -156,7 +156,14 @@
     python_bin = repository_ctx.os.environ.get(_PYTHON_BIN_PATH)
     if python_bin != None:
         return python_bin
-    python_bin_path = repository_ctx.which("python")
+    if repository_ctx.attr.python_version == "3":
+        python_bin_path = repository_ctx.which("python3")
+    elif repository_ctx.attr.python_version == "2":
+        python_bin_path = repository_ctx.which("python2")
+    elif repository_ctx.attr.python_version == "default":
+        python_bin_path = repository_ctx.which("python")
+    else:
+        _fail("Invalid python_version value, must be '2', '3', or 'default'.")
     if python_bin_path != None:
         return str(python_bin_path)
     _fail("Cannot find python in PATH, please make sure " +
@@ -377,6 +384,9 @@
         _PYTHON_BIN_PATH,
         _PYTHON_LIB_PATH,
     ],
+    attrs = {
+        "python_version": attr.string(default="default"),
+    },
 )
 """Detects and configures the local Python.
 
@@ -388,4 +398,10 @@
 
 Args:
   name: A unique name for this workspace rule.
+  python_version: If set to "3", will build for Python 3, i.e. will build
+      against the installation corresponding to the binary returned by
+      `which python3`.
+      If set to "2", will build for Python 2 (`which python2`).
+      By default, will build for whatever Python version is returned by
+      `which python`.
 """