Use Istanbul to show code coverage at coverage/lcov-report/index.html (#315)

It also prints it out after the tests like this:

```
=============================== Coverage summary ===============================
Statements   : 74.02% ( 1946/2629 )
Branches     : 54.6% ( 878/1608 )
Functions    : 75.5% ( 413/547 )
Lines        : 78.67% ( 1870/2377 )
================================================================================
```

Author: josephfrazier

.gitignore

@@ -1,5 +1,7 @@
 lib
 xregexp-all.js
+.nyc_output
+coverage
 
 # Compiled Python files
 *.pyc