Get correct version by git describe

When the pdc module is imported into another git repo, the version
reported is the version from that repo, not from PDC.

pdc/__init__.py

@@ -8,9 +8,11 @@
 
 VERSION = "0.5.0-0.1.beta"
 
+old_cwd = os.getcwd()
+os.chdir(os.path.dirname(os.path.dirname(__file__)))
 # NOTE(xchu): use `git describe` when under git repository.
 if os.system('git rev-parse 2> /dev/null > /dev/null') == 0:
-    pipe = subprocess.Popen("git describe",
+    pipe = subprocess.Popen("git describe --tags",
                             shell=True,
                             stdout=subprocess.PIPE,
                             stderr=subprocess.STDOUT)
@@ -31,6 +33,8 @@
     else:
         VERSION = version_list[0] + '-' + version_list[1].replace('-', '.')
 
+os.chdir(old_cwd)
+
 
 def get_version():
     """