Use stem to create test descriptors

Hi Isis. Stem now provides the ability to create and sign descriptors.

Unfortunately I didn't manage to get bridgedb's tests running but its use of leekspin looks pretty simple. Replacing it with Stem. There was a comment warning that descriptor creation takes a while but for what it's worth that doesn't seem to be the case. I'm running on an antiquated ten year old desktop and I can generate nineteen signed descriptors a second...

atagar@odin:~/Desktop/tor/bridgedb$ time ./scripts/create-descriptors 250 

real  0m13.087s
user  0m12.753s
sys 0m0.320s

atagar@odin:~/Desktop/tor/bridgedb$ cat test_descriptors/descriptor_15
router Unnamed2006509655 194.39.149.152 9001 0 0 
published 2005-06-15 02:26:44
bandwidth 153600 256000 104590
reject *:* 
onion-key
-----BEGIN RSA PUBLIC KEY-----
NGExNDZmMDU2YWRkYWUwODhiM2ZkNjg3NmNhNDM0YmMzYjc5ZmE5MDViNTRkYjA1
OGMwOGJmNTNmM2NiNzg3NTRjNTdjMDA1MmZlY2QxNzc5YTAwNTQ1OWYyNmRiN2Jm
YWFkM2RlNDRhOTQ1ZmRiZjExYTdmOTE4NzkyMTIwNTQ2YzY1NTAzNmM1MDM=
-----END RSA PUBLIC KEY-----
signing-key
-----BEGIN RSA PUBLIC KEY-----
MIGJAoGBALS1NGORgH3xEVOSd5Zs92z3bcrvjEK6WMYNkUiFUVvi7HhEFOT7vinP
vxUtPRakjDD7Kxg8LbL7PtyEtm29uSL9ggzZTZIHmMd4dIqAYSVfuN2wj/Stk5qp
2286BrmhuWyU/ZKLZXrz3g3JOkx9feJ9UxaxGeTGLK626WwMKuHjAgMBAAE=
-----END RSA PUBLIC KEY-----
router-signature
-----BEGIN SIGNATURE-----
pIEyuX8oUjMu2W1bqZ1KagpKDlRxMGBH8rupoWIuIjkvfBXC4goch9MHsa3S/Mf3
0Tb0PWS/KVFnQyTIPPIi5+iV5J/9ol87HPmrnCP4yP3NlE/nnlpbIOfuB+vdZv6n
BApDNLkWuuZCirNOqeps6BVBsH58v35nQoUY+HYOK60=
-----END SIGNATURE-----

Attached is a patch file you should be apple to apply with 'git am stem_descriptor_creation.patch'. Note that this requires the version of stem in the git repo to work. I say 'stem 1.6' in the output despite that version not yet being tagged since that's the next release.