Fix documentation typo

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branch : /fix-documentation-typo-1371479301909
Merged in RhysU/eigen//fix-documentation-typo-1371479301909 (pull request #25)

Fix documentation typo
Require at least cmake version 2.8.2 (bug 606).
Fix compilation issue with some compilers (when doing using Base::foo;, foo must be visible in the direct base class)