Implement known values of Si, Ci, Shi, Chi at oo and -oo
Tests for values of Si, Ci, Shi, Chi at oo and -oo
Replace 0 by S.Zero
Add Fresnel functions to the docs
Merge pull request #1662 from raoulb/fix_issue_3503

Fix issue 3503
Merge pull request #1705 from raoulb/fresnel_doc

Add Fresnel functions to the docs

Merging this now.

I eventually plan to improve the sections of the "special" functions page after the new "erf*" functions are in.