Issue313

Fixes #313 

@gregcaporaso can QIIME config handle using environment variables? https://github.com/wasade/qiime/pull/new/issue313#L8L52

The changes essentially boiled down the following, with the exception of adding lines to define ``$QIIME_DIR`` where appropriate in the tutorials.

```bash
cd doc/install
sed -i '' 's/\/home\/qiime/\$HOME/g' *.rst
cd ../tutorials
sed -i '' 's/\/home\/qiime\/qiime_software/\$QIIME_DIR/g' *.rst
sed -i '' 's/\/home\/ubuntu\/qiime_software/\$QIIME_DIR/g' *.rst
sed -i '' 's/\/home\/ubuntu/\$HOME/g' *.rst
sed -i '' 's/\/home\/qiime/\$HOME/g' *.rst