Implement assertion macros for ApproxEq

These macros yield readable error messages as test assertions for ApproxEq
types. They can be invoked as:

    assert_approx_eq!(a, b);
    assert_approx_eq_eps!(a, b, eps);

Fixes #40.
Merge pull request #43 from aepsil0n/assert_approx_eq_macros

Implement assertion macros for ApproxEq
Make `from_array`-like methods static.

Fix #45.
Add float constants to `BaseFloat`.

Fix #42.
Replace the double dispatch trick by multidispatch!

Fix #38.
Merge pull request #49 from sebcrozet/multidispatch

Replace the double dispatch trick by multidispatch!
Do not mantion the double-dispatch on the doc.
Update to the last rust-nightly.

Version of rustc: 0.13.0-nightly (fac5a0767 2014-11-26 22:37:06 +0000).