Added a test for Math.erf
The added test now passes.
Added test for erfc
test for erfc passes
Added test for lgamma

Apparently the first part of the test already worked so the interesting one is
the second one: Math.lgamma(-1) should return [infinity, 1].
Implemented lgamma(-1)

Along the way I noticed some errors in my test for this feature. So the correct
test is also a part of this commit.
Added a test for lgamma(NaN)
Fixed buggy test

The test I added previously is incorrect because in Python the equation
"float('NaN') == float('NaN')"
returns False.
I use math.isnan now to check whether a float represents NaN.
Fixed the lgamma(NaN) issue

rubyspec/core/math/lgamma_spec.rb can be untaged with --untranslated now.
Using rfloat.erf[c] instead of math.erf[c]

rfloat is imported from rpython.rlib
Therefor I hope that it won't let the translation crash.
Using RPython inf and NaN instead of Python ones
Found a better way to let the lgamma-specs pass

The first components of lgamma(-1) and lgamma(NaN) are actually calculated
correctly by rfloat.lgamma. It's the second components that cause the spec to
fail. So in the current version only the calculation of the second component is
modified.
Merge remote-tracking branch 'topaz/master'
untag passing Math specs
Added myself to the authors
Removed unused import RubyError
Implement Hash#select!
Implement Hash#keep_if
Implement Hash#select
Implement Hash#reject!