set precision of TCPCollector's metrics to 2

[ikruglov]

Having precision=0 means that any non frequent metrics will be hidden by
diamond. For example: if interval=60 and a counter incremented once a minute
we have following math done by derivate():

old=1
new=2
derivative_x = 2-1 = 1
derivative_y = 60
result = 1 / 60 = 0.01666666666667

this value is later passed to Metric class which does following if
precision=0:

value = sprintf("%.0f", 0.01666666666667)

as result value becomes 0.

[coveralls]

Coverage remained the same at 24.127% when pulling e644b91 on ikruglov:master into 746ed8e on python-diamond:master.

[shortdudey123]

Can you make this a config option instead of changing the hard coded value?

[shortdudey123]

Also update docs/collectors/TCPCollector.md with the precision config option