Closing improvements and several fixes

[weinrank]

This is a huge PR, sorry ... :)

Changes:

closing mechanism refactoring
If a peer closes the transport connection, this is now indicated by the on_close event and not by receiving no data when calling neat_read.
According to #316

callback handling
The callback handling has been improved in many aspects.
For example on_all_written is called every time a neat_write call has successfully delivered data to the transport layer and the flow buffer is drained.
See #272

improved memory management
The library should now handle memory a lot better, valgrind is much happier now. :)

==15298== 
==15298== HEAP SUMMARY:
==15298==     in use at exit: 0 bytes in 0 blocks
==15298==   total heap usage: 61,819 allocs, 61,819 frees, 6,215,102 bytes allocated
==15298== 
==15298== All heap blocks were freed -- no leaks are possible
==15298== 
==15298== For counts of detected and suppressed errors, rerun with: -v
==15298== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

misc changes

• RUN_ONCE and many flows fails #291 should be fixed
• examples are not installed systemwide by default
• improved OpenSSL detection
• examples cleaned (e.g. http client supports https, https client removed)
• message preserving/stream property
• code cleaning

Comments welcome...