Improvements to the vector module: Del and Orienters

1. Introduces the Orienters API for good code structure
2. Del class now returns unevaluated expressions(vectorial or scalar) composed of Derivative instances. The user can call .doit() to evaluate them.
3. adds the following printing implementations for the vector module - pretty, latex
4. scalar\_map method now takes into account the relative location of one coordinate system wrt another.
5. adds the methods 'measure\_numbers()' to Vector
6. Adds helper methods for calculating curl, divergence, gradient. Also adds the scalar\_potential and scalar\_potential\_difference functions present in the physics.vector module
No. I have made the axis orienter accept a coordinate system for its rotation\_matrix method. The user can just do <code>rotation\_matrix()</code> instead of <code>rotation\_matrix</code>. Though I have cached that method of the axis orienter, so that rotation matrices wrt the same system are computed again and again.