added native "TSC_IsValidChannel"

it should be used in combination with "TSC_GetChannelUserLimit" and "TSC_GetChannelRequiredTP", because they both return valid values ('0') when an invalid channel id is used
improved PAWN include

channel types and kick types are now tags
this should prevent users from using raw numbers instead of the correct enum definitions
massive code improvements

- this plugin now uses C++11 features
- there are also some small bugfixes
- this change effectively increases the required compiler version on Linux to 4.8