Feature: Added support for generating logarithmic units

- "Logarithmic" and "LogarithmicScalingFactor" objects are used in the json file to indicate the unit is logarithmic. These objects are optional and assumed to be false if not specified.
- Added a separate PowerShell script to generate the logarithmic arithmetic operator source code and logarithmic arithmetic operator unit test source code.
- This lays the groundwork for being able to auto-generate logarithmic units like any other unit. For example, it should be possible to auto-generate a decibel unit as well as any sub-unit specified [here](http://en.wikipedia.org/wiki/Decibel#Suffixes_and_reference_values) using this code. I've tested it on voltage ratio--as seen below with units of dBV, dBmV, dBuV-- and power ratio and it works nicely.
- I'm adding this feature so that I can next implement `Level` (with units of decibels), `ElectricPotentialRatio`, and `PowerRatio`. I'm not sure if you want these units in your library however, since they're specialty units related to RF, electronics, and audio engineering (this is the area I work in which is why I'd like to add them).

----------

Example:
```json
{
    "Name": "ElectricPotentialRatio",
    "BaseUnit": "DecibelVolt",
    "Logarithmic" : "True",
    "LogarithmicScalingFactor" : "2",
    "XmlDoc": "The strength of a signal expressed in decibels (dB) relative to one volt RMS.",
    "Units": [
        {
            "SingularName": "DecibelVolt",
            "PluralName": "DecibelVolts",
            "FromUnitToBaseFunc": "x",
            "FromBaseToUnitFunc": "x",
            "Localization": [
                {
                    "Culture": "en-US",
                    "Abbreviations": ["dBV"]
                }
            ]
        },
        {
            "SingularName": "DecibelMicrovolt",
            "PluralName": "DecibelMicrovolts",
            "FromUnitToBaseFunc": "x - 120",
            "FromBaseToUnitFunc": "x + 120",
            "Localization": [
                {
                    "Culture": "en-US",
                    "Abbreviations": ["dBµV"]
                }
            ]
        },
        {
            "SingularName": "DecibelMillivolt",
            "PluralName": "DecibelMillivolts",
            "FromUnitToBaseFunc": "x - 60",
            "FromBaseToUnitFunc": "x + 60",
            "Localization": [
                {
                    "Culture": "en-US",
                    "Abbreviations": ["dBmV"]
                }
            ]
        }
    ]
}
```