argcomplete: FastFileCompleter that doesn't call bash in subprocess, strip prefix dir

```
timeit result for 10000 iterations of expanding '/d' (lowered the count in the code afterwards)
#                      2.7.5     3.3.2
# FilesCompleter       75.1109   69.2116
# FastFilesCompleter    0.7383    1.0760
```
- does not display prefix dir (like bash, not like compgen), py.test /usr/<TAB> does not show /usr/bin/ but bin/

bench/bench_argcomplete.py

@@ -0,0 +1,19 @@
+
+
+# 10000 iterations, just for relative comparison
+#                      2.7.5     3.3.2
+# FilesCompleter       75.1109   69.2116
+# FastFilesCompleter    0.7383    1.0760
+
+
+if __name__ == '__main__':
+    import sys
+    import timeit
+    from argcomplete.completers import FilesCompleter
+    from _pytest._argcomplete import FastFilesCompleter
+    count = 1000 # only a few seconds
+    setup = 'from __main__ import FastFilesCompleter\nfc = FastFilesCompleter()'
+    run = 'fc("/d")'
+    sys.stdout.write('%s\n' % (timeit.timeit(run,
+                                setup=setup.replace('Fast', ''), number=count)))
+    sys.stdout.write('%s\n' % (timeit.timeit(run, setup=setup, number=count)))