Changes to cvxpy interface

[Steven Diamond]

I've been contemplating a few changes to the cvxpy interface.

1. Change semidefinite to Semidefinite for consistency.
2. Change expr.size to expr.shape for consistency with NumPy. expr.size would then give rows*cols, like in numpy.
3. Raise DCP errors as soon as a non-DCP expression is formed.

There are a few more that I can't remember right now. I'll add them in later.

Any comments would be appreciated.

[Steven Diamond]

Also, change the solver statuses OPTIMAL and OPTIMAL_INACCURATE to SOLVED and SOLVED_INACCURATE because OPTIMAL_INACCURATE is kind of an oxymoron.