Convert log likelihood values from mnl_estimate to Python floats.
making changes to support reading and writing yaml

This is for compatibility with urbansimd
adding list_models and exec_model
pep8 errors resolved
Merge pull request #24 from synthicity/urbansimd-yaml

Urbansimd yaml
Make rate column in relocation rates table an optional argument.
Function for comparing DataFrames in tests

Doesn't depend on row/column ordering and can handle null values.
Use new assert_frames_equal function in LCM YAML test.
Merge pull request #25 from synthicity/misc-upgrades

Relocation model change and data frame test function

- Make rate column in relocation model tables an optional parameter.
- Add a new testing utility function for comparing data frames independent of their row and column 
  ordering (that is, compare by label only).
  - This is useful for comparing DataFrames before/after YAML serialization to dictionaries
    because that loses all ordering.
Add YAML IO methods for RelocationModels.