made reduce_along_dim preserve dtype by default (as discussed in #92)
renamed dim method parameters to dimen to avoid NMatrix#dim naming confusion; added explicit call-seq blocks to mapping/reduction methods
Merge branch 'master' of git://github.com/SciRuby/nmatrix into nmatrix-reduce-type-fixes

Conflicts:
	lib/nmatrix/nmatrix.rb
Merge pull request #94 from cjfuller/nmatrix-reduce-type-fixes

made reduce_along_dim preserve dtype by default (as discussed in #92)