DOC more complete coverage of API linking in examples

Previously ignored non-plot files.

Avoids some link over/undergeneration
TST add test for name identification in RST generation
TST avoid nose running setup() in gen_rst.py
TST add sphinxext to testing
TST remove dependencies for testing sphinxext
Merge pull request #3165 from jnothman/links_in_examples

DOC more complete coverage of API linking in examples