Need way for resolving complex in BigFracPwr

[mcreng]

BigFracPwr should originally be used in real only cases, yet there are cases which negative base and fraction expn (with even denominator) may yield complex number.
I need to find a way to:

• throw exception when evaluating it in real mode
• allow further calculation/resolve in BigCmplxFrac class
• beware of complex root multiplication (e.g. sqrt(-2) * sqrt(-2) should not yield 2 but -2)

[mcreng]

@danvim should complex number issues be put under Axiom build though?

[danvim]

@mcreng If you don't think it's easy enough to implement, then we can assign it to our next milestone. I'll make a new one.

[mcreng]

First two done mostly under the structure provided in commit 5ed41a6.
There are two methods for evaluating its approx value, one to BigDecimal and one to BigCmplxDecimal. BigReal would use the method for BigDecimal, and would check if its BigRealExact is complex upon construction of object, hence resolving the first point. Since BigRealExact would not throw exception immediately when a complex is constructed in it, it can be passed to BigCmplxExact for further evaluation.