FEAT: add new solve_with_hint method.

[cournape]

This new method uses exactly the same algorithm as solve, but when a satistifiability error occurs, it computes a minimal unsatistiable set of jobs, which helps giving a more human-readable error message.

As an example, let's say we have a small dependency problem with the following packages:

mkl_11_3_1 = P(u"mkl 11.3.1-1")
mkl_2017_0_1_1 = P(u"mkl 2017.0.1-1")
mkl_2017_0_1_2 = P(u"mkl 2017.0.1-2")

numpy_1_10_4 = P(u"numpy 1.10.4-1; depends (mkl ^= 11.3.1)")
numpy_1_11_3 = P(u"numpy 1.11.3-1; depends (mkl ^= 2017.0.1)")

scipy_0_17_1 = P(u"scipy 0.17.1-1; depends (mkl ^= 11.3.1, numpy ^= 1.10.4)")
scipy_0_18_1 = P(u"scipy 0.18.1-1; depends (mkl ^= 2017.0.1, numpy ^= 1.11.3)")

Then, trying to install both scipy 0.18.x and MKL < 12 cannot work. With solve, the error message is:

Conflicting requirements:
Requirements: 'scipy >= 0.18.0-0'
    Install command rule (+scipy-0.18.1-1)
Requirements: 'scipy >= 0.18.0-0' <- 'numpy ^= 1.11.3'
    scipy-0.18.1-1 requires (+numpy-1.11.3-1)
Requirements: 'scipy >= 0.18.0-0' <- 'numpy ^= 1.11.3' <- 'mkl ^= 2017.0.1'
    numpy-1.11.3-1 requires (+mkl-2017.0.1-1 | +mkl-2017.0.1-2)
Requirements: 'scipy >= 0.18.0-0' <- 'mkl ^= 2017.0.1' <- 'mkl'
    Can only install one of: (+mkl-11.3.1-1 | +mkl-2017.0.1-2)
Requirements: 'mkl < 12-0'
    Install command rule (+mkl-11.3.1-1)

With the new solve_with_hint method:

The following jobs are conflicting:
    install scipy >= 0.18.0-0
    install mkl < 12-0

[cournape]

2 for the price of 1 ! Thanks