Add single stat for timing sections of code

[dannyOhNo]

Add single stat for timing sections of code, but without probabilistic calculations saved.

Given a high throughput system, the need may arise to time sections of code without performing probabilistic calculations (PmfStat).

The purpose of this is two-fold.

1) To reduce the processor load by eliminating the PMF calculations.

2) To reduce the data storage required by storing the PMF results.

To solve this need, I’ve done the following:

• Abstract PMF’s TimeManager class to BaseTimeManager
• PMF’s TImeManager now inherits from BaseTimeManager, overriding the __init__ and __exit__ functions.
• Abstract PmfStatDict to StatDict. PmfStatDict now inherits from StatDict
• Create new class, DoubleStat that contains the same timing functionality as PmfStat. This class returns the aforementioned StatDict (taken from PmfStatDict).
• Add value field to the PmfStatDict so that along with the probabilistic calculations, the raw value is preserved also.

[dannyOhNo]

All tests pass locally by doing...

source venv/bin/activate python setup.py test

Not sure why this is failing. If PR is going to be accepted, I can write some unit tests for the new class.