Provide means for invoking natives dynamically (issue #54)

Add two new functions:

- sampgdk_call_native(AMX_NATIVE native, cell *params)
- sampgdk_invoke_native(AMX_NATIVE native, cosnt char *format, ...)

where format specifies the types of the arguments (like in SetTimerEx and
similar functions). The following argument types are supported:

'i', 'd' - int
'b' - bool
'f' - float
'r' - const reference ('const cell *' in C)
'R' - non-const reference (output argument) (the C type is 'cell *')
's' - input string ('const char *' in C)
'S' - output string ('char *'), size must be passed in next argument