Inclusion of aliases for sign git commits and tags

To test this PR with GPG installed and configured:

1. Change any file and stage it
2. *gcs -m "sign my commit"*

Expected output

```
You need a passphrase to unlock the secret key for
user: "Bruno Oliveira da Silva <bruno@abstractj.org>"
2048-bit RSA key, ID 84DC9914, created 2012-04-18

[myproject 672ba9c] sign
 1 file changed, 1 insertion(+), 1 deletion(-)
```

3. *git log* and get the commit id
4. *gsps 672ba9cfa1566d1380f47b36acade5c04fcd249c*

Expected output:

```
commit 672ba9cfa1566d1380f47b36acade5c04fcd249c
gpg: Signature made Sat Aug  9 12:32:12 2014 BRT using RSA key ID 84DC9914
gpg: Good signature from "Bruno Oliveira da Silva <bruno@abstractj.org>"

    sign my commit

diff --git a/README.md b/README.md
index 9e760d7..4b14553 100644
--- a/README.md
+++ b/README.md
@@ -1 +1 @@
```

5.  *gts -m "Sign my tag" 0.2.1*

Expected output

```
You need a passphrase to unlock the secret key for
user: "Bruno Oliveira da Silva <bruno@abstractj.org>"
2048-bit RSA key, ID 84DC9914, created 2012-04-18
```

6. *gvt 0.2.1*

Expected output

```
gpg: Signature made Sat Aug  9 12:33:06 2014 BRT using RSA key ID 84DC9914
gpg: Good signature from "Bruno Oliveira da Silva <bruno@abstractj.org>"
```