Logging API enhancements

1. Rename sampgdk_log_warn to sampgdk_log_warning to match the actual
log level name (consistency).

2. Get rid of the generic sampgdk_log_message and introduce
sampgdk_log_info instead.

Now there are 4 logging levels: info, trace, warning, error.
Add ability to control logging verbosity
Slightly change log message format

Old format: [sampgdk] level: message
New format: [sampgdk:level] message
More logging
Add missing BODY_PART_* constants

Though they are kind of unofficial:

http://wiki.sa-mp.com/wiki/Body_Parts

Fixes #111.