Use clearer file and directory names for build output

This makes it easier to differentiate between the archives for
dependencies and that of the final build output by bob when sifting
through `/tmp/`. The archive type is now also apparent when reading
the console output.

Before:
```
Building formula foo in /tmp/bobrmyuSG:
    ...
Archiving.
/tmp/tmp4JUwdj
Deploying.
...
```

After (when combined with #28):
```
Building formula foo in /tmp/bob-rmyuSG:
    ...
Archiving.
Created: /tmp/bob-build-tmp4JUwdj.tar.gz
Deploying.
...
```

bob/models.py

@@ -124,7 +124,7 @@ class Formula(object):
                     sys.exit(1)
 
                 # Grab the Dep from S3, download it to a temp file.
-                archive = mkstemp()[1]
+                archive = mkstemp(prefix='bob-dep-', suffix='.tar.gz')[1]
                 key.get_contents_to_filename(archive)
 
                 # Extract the Dep to the appropriate location.
@@ -139,7 +139,7 @@ class Formula(object):
         self.resolve_deps()
 
         # Temporary directory where work will be carried out, because of David.
-        cwd_path = mkdtemp(prefix='bob')
+        cwd_path = mkdtemp(prefix='bob-')
 
         print 'Building formula {} in {}:'.format(self.path, cwd_path)
 
@@ -158,7 +158,7 @@ class Formula(object):
 
     def archive(self):
         """Archives the build directory as a tar.gz."""
-        archive = mkstemp()[1]
+        archive = mkstemp(prefix='bob-build-', suffix='.tar.gz')[1]
         archive_tree(self.build_path, archive)
 
         print archive