use-agent doesn't work with gpg2

[Ross Williams]

If use-agent is specified and gpg2 is installed and being used by duplicity, the gpg binary fails to get the password from gpg-agent. This appears to be caused by the --passphrase-fd option, which forces gpg to get the passphrase from duplicity instead of from the agent. This is in contrast to GnuPG 1.x behavior, when use-agent took priority over passphrase-fd. I've submitted a proposed fix in a related branch.

Duplicity 0.6.15
Python 2.7.2
Mac OS X 10.7.1

Log output:
GPG error detail: Traceback (most recent call last):
  File "/opt/local/bin/duplicity", line 1333, in <module>
    with_tempdir(main)
  File "/opt/local/bin/duplicity", line 1316, in with_tempdir
    fn()
  File "/opt/local/bin/duplicity", line 1286, in main
    full_backup(col_stats)
  File "/opt/local/bin/duplicity", line 455, in full_backup
    globals.backend)
  File "/opt/local/bin/duplicity", line 333, in write_multivol
    globals.gpg_profile, globals.volsize)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/duplicity/gpg.py", line 298, in GPGWriteFile
    file.write(data)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/duplicity/gpg.py", line 168, in write
    self.gpg_failed()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/duplicity/gpg.py", line 180, in gpg_failed
    raise GPGError, msg
GPGError: GPG Failed, see log below:
===== Begin GnuPG log =====
gpg: no default secret key: Bad passphrase
gpg: [stdin]: sign+encrypt failed: Bad passphrase
===== End GnuPG log =====

Removing still remembered temporary file /Users/XXXX/.cache/duplicity/duply_test/duplicity-rGhhbk-tempdir/mktemp-SYqbzW-1
Removing still remembered temporary file /Users/XXXX/.cache/duplicity/duply_test/duplicity-udfnfS-tempdir/mktemp-hssitk-1
13:09:58.000 Task 'FULL' failed with exit code '31'.
--- Finished state FAILED 'code 31' at 13:09:58.000 - Runtime 00:00:00.000 ---