Add support for StrEnum

[chrisjbremner]

Enum allows you to subclass it with additional mixins to create custom Enums that act in a similar fashion to what you are mixing in. For example, you can create an IntEnum which allows the enum to be used like an int. Similarly, you can make a StrEnum to do string comparisons.

Using a StrEnum almost works with the existing code base, but there's one small if statement that incorrectly catches StrEnums but not regular Enums, which I fix in this PR. I also included a test that would fail without this code change, but succeeds after.

[coveralls]

Coverage decreased (-0.2%) to 98.333% when pulling c4dae18 on chrisjbremner:master into 02076d3 on pytransitions:master.

[chrisjbremner]

@aleneum I also found that the diagramming extensions did not work when using Enums for states (because it sometimes referred to each state by the Enum's name, and other times by just the Enum itself), so I attempted to fix that as well in my most recent commit.

[chrisjbremner]

Looks like my second commit broke some tests, I will fix that when I get a chance.

[chrisjbremner]

Closing, as this was fixed in #357