fix isless and cmp/lexcmp for floating point

for now cmp() uses the total ordering, but we might change it to give a
DomainError for NaN arguments
make nextfloat/prevfloat saturate at Inf, as per the IEEE standard

ref #5290
test/euler.jl: slightly faster euler #4 algorithm
Update manual to document sinpi(x) and cospi(x)
Fast sum along dimensions over contiguous arrays
specialized for cases with region being an integer
Provides diag(M::SymTridiagonal) and diag(M::Tridiagonal)
Fix #5298
rename: radians2degrees => rad2deg, degrees2radians => deg2rad.

These seem to be fairly industry-standard names.

NumPy:

    http://docs.scipy.org/doc/numpy/reference/generated/numpy.rad2deg.html
    http://docs.scipy.org/doc/numpy/reference/generated/numpy.deg2rad.html

PHP:

    http://www.php.net/manual/en/function.rad2deg.php
    http://www.php.net/manual/en/function.deg2rad.php

Other systems don't seem to ship with functions for this.
Merge pull request #5300 from JuliaLang/sk/rad2deg

rename: radians2degrees => rad2deg, degrees2radians => deg2rad.
some tuning of the vector kernels
allow specify the result type of sum.
allow minimum and maximum over a subvector
Add Rmath notes to DISTRIBUTING.md
apply the new reduction framework to maximum & minimum
apply new reduction framework to prod
updates to the maximum & minimum codes
more flexible code generation functions

Now the code-gen allows generating reduction functions that may contain more than one input arrays.
base/complex.jl: remove pointless promotion rule.

I'm pretty sure that there's no way to actually invoke this rule.
Merge pull request #5294 from lindahua/dh/reducedim2

WIP: Fast implementation of reduction along dims