Fix for android C++ stdlib not having std::to_string.
Use convention to reduce repetition.
Rename friend-not-found error codes.

All error codes that mean "friend number didn't designate an existing
friend" are now called FRIEND_NOT_FOUND.
add few tests for send message
add few tests for send action
minor fix of send action test
Renamed some functions for more consistency.
Rethrow protobuf exceptions.

Exceptions in this place would mean something very bad happened, and we
want to know about that.
"nospam" is actually a word in the Tox world.
nullptr is a valid input (causes error) for some functions
Protobuf source code reformatting.
Removed .keep file now that there are actual tests.
Consistency in comments
Added a bunch of error path tests.
Add very simple beginning of client library + spec.

And a dummy Main that does nothing for now.
Change specification of effect of sending less than requested.
Merge branch 'sk8ter-master'
Remove broken tests
Merge branch 'pippijn-master'

Conflicts:
	src/test/java/im/tox/tox4j/ToxCoreTest.java
Fix sign-compare warning for tox_file_data_size