Allow escaped characters in shell

Per #708.

This patch allows escaped characters in the shell, and makes the
apostrophe a quotation mark. The escape character is backslash.

The term "escape character" is used liberally in here: if a backslash is
encountered in the command line, the next character will be taken
verbatim. No escape sequences are understood, i.e. `"\n"` is just the
letter `n`, not a new line.
native/posix: Don't include sys/types.h in sem

`sys/types.h` contains the native definition for `pthread_*_t`. This
causes clashes if you want to use `semaphore` and `pthread` in the same
application.
ccnl: added documentation and proper event handling for dropped packets
posix: added sleep and usleep
test: added test case for posix sleep
Merge pull request #839 from mehlis/posix-sleep

posix: added sleep and usleep
Merge pull request #832 from mehlis/ccn-lite-follow-up-11

ccnl: added documentation and proper event handling for dropped packets
Merge pull request #806 from Kijewski/sem-pthread-native

native/posix: Don't include sys/types.h in semaphore.h
Merge pull request #794 from Kijewski/shell-args-escape

shell: Allow escaped characters in shell