Introduced repo_base_branch tunable.

The goal of this addition is to make possible creating projects
which uses only one particular branch in the repository.
More straightforward way to generate local repository path.

Previously, if you set up two different repos with locations like
/data/hg/nginx-1.2 and /data/hg/nginx-1.3, repo_path for both of them
would be the same: /data/hg/nginx-1.

Now, we use the whole repo URL as the local repo_path, but with
all slashes ('/') replaced by underline characters ('_').
Honor revision's branch while obtaining logs from Mercurial repo.
Use repository base branch for displaying changes.
Use repository base branch for displaying timeline data.
Replace semicolons and dots by underline sign in local repo_path.
Expand maximum benchmark name length to 60 chars.
Add repo_base_branch field to project management admin page.
Include project name in local repo_path.

This may help if more than one projects are configured
with the same repository URL, but having different default
branches.
Use DAG range in getting Mercurial logs.
Increase revisions limit in changes view.
Add tags field to Mercurial log template.
Change revision's date format.
Save revision's tag to database.
Sort executables by revision date (reversed) in comparison.
Do not store 'tip' tag for Mercurial changesets.
Fix links to the timeline view in the changes table
Pin Django dependency to 1.4
Fixes #135
Avoid future South version breakage
Merge remote-tracking branch 'defan/master'

Conflicts:
	codespeed/models.py
	codespeed/views.py
	requirements.txt
	setup.py