Documented how the pieces fit together.

There are two main pieces:

* L-BFGS-B library with C API but written in Fortran.
* Go package providing Go API for library.
Added copyright notices to all files that needed one.
Documented lbfgsb_minimize (C API).
Improved, reformatted C API documentation.
Converted error handling to general status handling.

* Renamed most error* to status*.
* Updated enumerated status codes to agree with optim.go and have prefix
  appropriate for C.
* Added subroutine to translate L-BFGS-B tasks to unambiguous, disjoint
  states that fit better with status codes.
* Reworked logic to use translated states instead of raw tasks.
Removed unnecessary array copies.

* Array copies may need to be restored (to do the necessary conversions)
  if it cannot be guaranteed that the Fortran and C types are always
  binary compatible.
Ensured null termination of C strings in Fortran.
Converted error handling to status handling in Go package.

* Renamed error* to status*.
* Updated buffer length now that Fortran ensures proper handling of C
  strings and thus can use the whole buffer.
Implemented remainder of Lbfgsb.Minimize.

* Implemented Lbfgsb struct members.
* Implemented argument checking.
* Implemented parameters and overriding.
* Implemented construction of bounds control array.
* Cleaned up and organized existing code (e.g. status message).
* Needs refactoring to reduce repeated code.