Add ability to use a custom axis with jackknifing

A custom axis can now be added to jackknifed plots,
preprocess_coords_file is now updated to handle this type of data
requests. The test suite was updated accordingly.

The script interface will no longer disallow a custom axis and
jackknifed plots. However no more than one custom axis can be used and
that's still forbidden by the script interface.
Fix script interface for jackknifing

Checking the number of custom_axes was failing when None was specified,
this has been fixed in this commit.

Thanks for catching this one @antgonza.
Merge pull request #39 from ElDeveloper/issue_11

Add jackknifing functionality -- fixes issue 11
Merge branch 'master' of git://github.com/qiime/emperor into issue_34