[Issue 180] Fix python2 x python3 incompatibility

[almostimplemented]

Addresses bug #180 encountered when running Graphite in Python 3.

[brutasse]

Would it be possible to add a regression test? I'm not sure which conditions triggers this issue.

[almostimplemented]

Sure, added.

The condition to trigger is when the operands of the compare in bisect.bisect_left (specifically: if a[mid] < x: lo = mid+1) agree on the first value of the tuple, thus requiring a comparison of the next entry. In the current code, that entry is None, which is no longer a supported operand for comparison in Python3.

[almostimplemented]

Hm, it looks like this passed the automated test... I arrived upon this value by iterating through metric values (e.g. for x in range(100): ... 'hosts.worker%s.cpu' % x ... ). This was the first value triggering the raised bisect exception.

[RincewindsHat]

This seems to work for me.
To make this better findable, this is the error message I got :

TypeError: '<' not supported between instances of 'tuple' and 'NoneType'