Added $lineheightmod to ktype, Added kunit function

Added $lineheightmod parameter to the ktype mixin, to replace push and pull in the future with a single value. It takes negative and positive integers like the scale unit, corresponding to half-line modificaitons to the lineheight of the output typographic element. eg. use ktype(0,1,0,2) to add a whole baseline unit to the lineheight.

Added kunit function for non-type vertical rythm arrangements, simply declare kunit(X) to output X baseline height in pixels, it also accepts a second [boolean] parameter to output in rem units instead. eg. kunit(2, true) for 2 baseline units equivalent in rems. This is useful for spacing and sizing your layout units and rapid prototyping of frameworks.
Streamlined calculateRem / krem, incorporating stripUnits to accept non px size declarations

included a stripUnits function as suggested by https://github.com/ronilaukkarinen on https://github.com/Pushplaybang/knife/issues/3. This is originally an excerpt from https://github.com/sass/sass/issues/533 by https://github.com/robwierzbowski

The suggestion on https://github.com/Pushplaybang/knife/issues/3 broke the ie/px fallback, printing unitless units like the following, so I plugged in stripUnits there as well, and added the px suffix back in
```
font-size:16;
font-size:1rem
```

Streamlined calculateRem while I was at it
Merge remote-tracking branch 'origin/patch-1'
enabled lineheightmod for 0 scale ktype + fix for 'may only compare numbers' using libsass

defaulted push and pull to '0' rather than 'false' in ktype() mixin, in which @if $pull != 0 comparison is made later, which might be libsass. The bug might be unrelated but the commit is just logical anyhow, no need to false push/pull, just use the intended numerical.

Also enabled lineheightmod for 0 scale ktype, in case one wants double spaced body sized type for example.
Merge pull request #8 from whatsnewsisyphus/master

Added $lineheightmod to ktype, Added kunit function with minor fixes and enhancements.