Let users select from different buffer sizes.

[256dpi]

As the underlying library is using templates to manage buffers we cannot just use any size dynamically.
To allow some freedom, we can integrate specific sizes the user can choose from:

client.use64Bytes();
client.use128Bytes(); // default
client.use256Bytes();
client.use512Bytes();

[256dpi]

Another possible solution would be to provide a AdvancedMQTTClient that uses template variables for advanced customization and usage:

AdvancedMQTTClient<256> client;

[markrad]

+1 for this. The buffer is too small for my purposes.

[256dpi]

This is beeing addressed in #59.