state requirements before attempting install?

travis is failing to make the *.c files first - maybe stating the requirements
first will help
handle failure to import scipy

Calling functions that rely on scipy will result in an error. These tend to be
specialty modules, and don't include core functionality.
raise CRSError if trying a lon-lat azimuth without pyproj
Verify that the source files for extension modules in place, and if not, abort building extensions
Merge branch 'travis'