Merge branch 'master' of git://github.com/qiime/qiime
Retrieve qiime_scripts_dir on the fly

If the paramenter qiime_scripts_dir is not set in the qiime_config file,
then the directory is inferred from the location of
print_qiime_config.py.

Fixes #881
Change all_tests.py to use get_qiime_scripts_dir()
Merge pull request #930 from ElDeveloper/issue_881

Retrieve qiime_scripts_dir on the fly