Declare PyChart package name in requirements.txt

While calling `pip install -r requirements.txt`, the PyChart tarball is
always downloaded even if `pip` has a download cache configured (which
it does by default in later versions).

This is because `pip` has no way of knowing which package the PyChart
tarball contains before downloading and running its `setup.py`.

By declaring the PyChart "egg" in `requirements.txt` the tarball is no
longer downloaded if it's already in the download cache.

requirements.txt

@@ -3,7 +3,7 @@ Jinja2==2.7.3
 Mako==1.0.0
 MarkupSafe==0.23
 Pillow==2.5.1
-http://download.gna.org/pychart/PyChart-1.39.tar.gz
+http://download.gna.org/pychart/PyChart-1.39.tar.gz#egg=PyChart
 PyYAML==3.11
 Werkzeug==0.9.6
 argparse==1.2.1