Adds get_build_path to BuildableTemplateView, which can handle a build_path property set to reverse_lazy

[mattaustin]

I've added a get_build_path method to BuildableTemplateView. This coerces the build_path property to text, and strips any preceding '/', which allows us to set build_path to reverse_lazy values. This partially resolves issue #120 (we're not handling 'directory' paths yet).

There is some related further work I'd like to complete, but I didn't want to create a huge diff in one pull request. If this gets accepted, I'll continue work on:

• Handling build_path (and reverse_lazy) values which do not include an explicit filename (similar to how BuildableDetailView works), e.g. handling a value of "/about/", without having to specify "about/index.html". This would include a setting to allow an alternative "default" filename, rather than hard-coding 'index.html' in python code in multiple places.
• Eventually moving common "build_path" functionality from Buildable[Template/Detail/List/Date]Views to the base "BuildableMixin".

[coveralls]

Coverage remained the same at 79.845% when pulling 2bd6ceb on mattaustin:reverse_lazy into 18bbb24 on datadesk:master.

[palewire]

I like your incremental approach here. I know there are people out there, including myself, who have implemented dynamic build paths already with properties like

@property
def build_path(self):
    # Do python blah blah
    return path

I think this upgrade will be backwards compatible with that pattern. I'd like to confirm that. Once I do, I think we can merge this.

[palewire]

Sorry this took me so long to merge. It is in master and will ship to PyPI with a new version once it gets past the TravisCI tests with my other changes.