Make API includes consistent.

It is "convenient" when header files can be referenced in [include statements](http://en.wikipedia.org/wiki/Include_directive#C "Inclusion of other files") without the specification of a directory. But a few file names (like "[exception.hpp](https://github.com/dermesser/libsocket/blob/96811c50d4008a86c0e3994f404f0baf75080bea/headers/exception.hpp#L1 "Is such a file name popular?")") were chosen for C++ source files here that have got the potential to get in conflict with name selections from other software libraries. The name "libsocket" was therefore [added as a parent directory](http://en.wikipedia.org/wiki/Namespace#In_programming_languages "Addition of a name space") to some includes. I hope that this change makes their usage consistent.

The affected source code places were also adjusted so that [angle brackets](http://stackoverflow.com/questions/21593/what-is-the-difference-between-include-filename-and-include-filename "What is the difference between #include <filename> and #include “filename”?") are used there now.

Two variables were corrected so that they fit to [Make naming conventions](http://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html "variable selection") now.
The variable "API_PARENT_INC_DIR" was introduced for internal use in Make scripts.


This update suggestion corresponds to the issue "[Clarification for header file references](https://github.com/dermesser/libsocket/issues/11 "Check paths for includes")".