add hierarchy to Benchmark model

    - Sometimes benchmarks have a big number of single results. It is
      favourable to keep them all. Thus, they might be organised in an
      hierarchical way.
      Example: To evaluate computer systems there might be one
      overall benchmark number. That number is calculated from sub
      benchmarks as CPU, hard disc I/O, RAM I/O, etc.
    - The single or the summarised result may be shown in the standard
      templates in a later version. For now all benchmarks are shown.

Change-Id: I47062535bc0ec6e1d25ae857b4af74f207737c37

Author: a8

codespeed/models.py

@@ -132,6 +132,8 @@ class Benchmark(models.Model):
     )
 
     name = models.CharField(unique=True, max_length=30)
+    parent = models.ForeignKey('self', verbose_name="parent",
+        help_text="allows to group benchmarks in hierarchies", null=True, default=None)
     benchmark_type = models.CharField(max_length=1, choices=B_TYPES, default='C')
     description = models.CharField(max_length=300, blank=True)
     units_title = models.CharField(max_length=30, default='Time')