Update the version in the docs from the package

Use pkg_resources to get the version of the pecan
package instead of maintaining the version string
in two places separately.

Change-Id: I0970ef262054347ed9964431130fd60fdf4e7112
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge pull request #214 from dhellmann/update-doc-version-automatically

Update the version in the docs from the package