Class-based implementation with autosql-to-pandas support

[nvictus]

New features

• Refactored bbi functions into a cdef class (functional API preserved)
• Added support to read and parse a bigBed's AutoSql schema definition
  • AutoSql scalar types are mapped to numpy/pandas dtypes
  • AutoSql listed/array types or other exotic types are treated as regular strings

API changes

• BbiFile.fetch_intervals now returns a dataframe with columns correctly parsed using the autosql schema

@alexpreynolds If there is a desire to keep the old functionality of fetch_intervals providing a tuple generator, let me know.
CC @keller-mark

[nvictus]

@alexpreynolds, fetch_intervals now returns an interval iterator instead of a dataframe if iterator=True, which should behave just like the generator did. For the function iterator=True by default; for the class method iterator=False by default, so your bigbed tile fetcher should work without modification.

I'll go ahead and merge this. If you have time, give it a try and file an issue if it breaks.

[alexpreynolds]

Thanks very much!