…e callback's result to import rules files.

…back_on_include

 * Limited lines to 80 chars
 * include callback is now using its own user_data and does not conflict with other callbacks
 * Copying include callback result to the stack before pushing it into the flex scanner
 * yara's memory management functions are now exposed through the API
 * Making the lexer reponsible for freeing the memory allocated by the include callback
 * Updating docs accordingly

* adding a callback to allow external code to free the memory used to return the include callback's result
* API: un-exposing yara's memory management functions
* making include callbacks the only way ton include files
* adding a default include callback to open files from the disk (mimicking original behavior)
* updating docs accordingly