Make math and 'almost' functions publicly available.

Exposes underlying assertion logic as functions that return numbers (`mean`,`sum`,`sdeviation`) or booleans (`almostEqual`, `deepAlmostEqual`). Primary use would be other chai plugins that are overriding some behavior.

Usage:
```javascript
require('chai-stats').almostEqual(3.14159,3,0);
// evaluates to true  
```