Add importstr, closes https://github.com/google/jsonnet/issues/14
Add importstr to documentation
Implement syntax sugar for optional else, closes https://github.com/google/jsonnet/issues/11
Update docs for optional else branch
Implement string indexing, closes https://github.com/google/jsonnet/issues/12
Merge branch 'else-sugar'
Merge branch 'importstr'

Conflicts:
	doc/libjsonnet.js