Extend the C API to support loss functions

This extends the C API to support loss functions. Both
user-supplied cost functions as well as the stock Ceres cost
functions (Cauchy, Huber, etc) are supported. In addition, this
adds a simple unit test for the C API.

Supporting loss functions required changing the signature of the
ceres_add_residual_block() function to also take a thunk for the
loss function.

Change-Id: Iefa58cf709adbb8f24588e5eb6aed9aef46b6d73