Implemented basic interval arithmetic capabilities

This commit implemented interval arithmetic as class IV in sympy.core.sets Now
sympy can:
- add and subtract intervals with intervals and Real Numbers
- Multiply and divide intervals with intervals and Real Numbers
- exponentiate an interval to some positive integer power
- substitute an expression with interval and evaluate the resulting interval
Allowed IV as argument in elementary functions

Added code to handle IV as arguments in elementary functions.
Following functions are now supported:
- exponential: exp, log
- trigonometric: sin, cos, tan, cot
- hyperbolic: sinh, cosh
Added documentation for IV