Multithreading smooth-out

[Xasin]

Hey there!
For my smart home project, as I expanded it, I began to notice a few odd behaviours of my I2C-connected components.

Upon analysis with my Salea, I noticed two things:

• There was an unnecessary write in front of read operations. SLA+W was sent, but no data. (No data was expected)
• Communications seemed to get mixed up with each other.

The former point was a simple manner of rewriting the read() code to not always execute the write function.
The second point was a lack of synchronisation between multiple threads, allowing different Threads to simultaneously write to @device, thusly confusing what data and address were sent where.

The mutex I added makes sure that only one Thread can ever access the @device between setting address, writing and reading. It can additionally be accessed and locked manually, to perform multiple consecutive read or write operations to single or multiple devices.

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This change is 

[Xasin]

Review status: all files reviewed at latest revision, all discussions resolved.

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i2c.gemspec, line 3 at r2 (raw file):

Gem::Specification.new do |s|
  s.name        = 'i2c'
  s.version     = '0.4.2-dev'

Change to the official version before publishing

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Comments from Reviewable

[andec]

Thank you!