Configurable client bufio reader/writer sizes

[mochi-co]

Addresses #189 by making the client bufio.Reader and bufio.Writer sizes configurable as server options.

The new default value for the reader and writer buffers is 2048. Different values can be configured as such:

server := mqtt.New(&mqtt.Options{
    ClientNetWriteBufferSize: 1024,
    ClientNetReadBufferSize: 1024,
})

[coveralls]

Pull Request Test Coverage Report for Build 4394616403

• 26 of 26 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 98.596%

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Totals
Change from base Build 4297329515:	0.002%
Covered Lines:	5128
Relevant Lines:	5201

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💛 - Coveralls