Enhancement/default txt encoding from config

[sablanchard]

addresses #79 by adding a default encoding parameter to the urbanaccess configuration object and allows for user to change encoding if needed. Solves issues where user machine encoding is different than expected txt encoding.

• adds txt_encoding parameter to config with 'utf-8' default. This parameter is used when txt files are read with functions in _standardize_txt().
• updates _txt_header_whitespace_check() to use encoding from config, updates function for use with py2 vs py3 since py2 does not support encoding parameter, update to only modify txt files that have whitespace issues, and added prints
• adds prints to _txt_encoder_check()
• adds unit tests for functions in _standardize_txt()

[smmaurer]

This seems worth putting out a point release for, right?

[sablanchard]

Thanks @smmaurer yes I think there are enough updates collected on dev so far to do another release.